Wiley, 2011), as well as in several others I have, and from from several locations around the web, are: Primary (1 ∘) carbon atom - bonded to one other carbon atom, e.g. H O C H X 2 C H X 3 Secondary (2 ∘) carbon atom - bonded to two other carbon atoms, e.g. H O C H (C H X 3) X 2. The magnitudes of such secondary isotope effects at the α-carbon are largely determined by the C α-H(D) vibrations.For an S N 1 reaction, since the carbon is converted into an sp 2 hybridized carbenium ion during the transition state for the rate-determining step with an increase in C α-H(D) bond order, an inverse kinetic isotope effect would be expected if only the stretching vibrations. This organic chemistry video tutorial explains how to identify primary, secondary, tertiary hydrogen atoms and quarternary carbon atoms as well as for alkyl. Primarycarbon (1ocarbon): A carbon directly bondedto just one other carbon group.
POOLS,FLUXES AND A WORD ABOUT UNITS
In order to understand how carbon is cycled and how atmosphericCO2 will change in the future, scientists must carefully study theplaces in which carbon is stored (pools), how long it resides there,and processes that transfer it from one pool to another (fluxes). Collectively, all of the major pools and fluxes of carbon on Earthcomprise what we refer to as the global carbon cycle.
As you might imagine, the actual global carbon cycle is immenselycomplex. It includes every plant, animal and microbe, everyphotosynthesizing leaf and fallen tree, every ocean, lake, pond andpuddle, every soil, sediment and carbonate rock, every breath of freshair, volcanic eruption and bubble rising to the surface of a swamp,among much, much else. Because we can't deal with that level ofcomplexity, scientists often describe the carbon cycle by lumpingsimilar objects or environments into simpler groups (forest, grassland,atmosphere, ocean) and focusing only on the processes that are mostimportant at the global scale (see GlobalCarbon Cycle Diagram). As you mightimagine, part of the trick is understanding just what those processesare.
The following section is a brief overview of some of the importantpools and fluxes in the global carbon cycle (and note that, in ourdiscussion, we will use the terms pool, stock and reservoirinterchangeably). But first, it’s worth taking a moment toconsider the numbers and units scientists often deal with. Because the quantities of carbon in the Earth’s major carbon pools canbe quite large, it is inconvenient to use familiar units such as poundsor kilograms. Instead, we use other units that are better suitedfor expressing large numbers. For example, a Petagram of carbon (Pg),also known as a Gigaton (Gt), is equal to 10^15 grams or one billiontonnes. A tonne, also known as a metric ton, is equal to onethousand kilograms (1,000 kg). Because one kilogram is equal to2.205 pounds, one metric tonne is the same as 2205 pounds. Takingthis further, we can see that one Petagram is equal to just about2,200,000,000,000 (or 2.2 trillion) pounds! Expressing this as 1Pg is much simpler than working with that many zeros. Now we willconsider carbon stored on Earth in four main reservoirs.
CARBON POOLS
Depending on our goals, the Earth’s carbon pools can be grouped intoany number of different categories. Here, we will consider fourcategories that have the greatest relevance to the overall carboncycle. Keep in mind that any of these pools could be furtherdivided into a number of subcategories, as we will occasionallydiscuss.
The Earth’s Crust: Thelargest amount of carbon on Earth is stored insedimentary rocks within the planet’s crust. These are rocksproduced either by the hardening of mud (containing organic matter)into shale over geological time, or by the collection of calciumcarbonate particles, from the shells and skeletons of marine organisms,into limestone and other carbon-containing sedimentary rocks. Together all sedimentary rocks on Earth store 100,000,000 PgC.Recalling that 1 Pg is over two trillion pounds, this is clearly alarge mass of carbon! Another 4,000 PgC is stored in the Earth’scrust as hydrocarbons formed over millions of years from ancient livingorganisms under intense temperature and pressure. Thesehydrocarbons are commonly known as fossil fuels.
Oceans: TheEarth’s oceans contain 38,000 PgC, most of which isin the form of dissolved inorganic carbon stored at great depths whereit resides for long periods of time. A much smaller amount ofcarbon, approximately 1,000 Pg, is located near the oceansurface. This carbon is exchanged rapidly with the atmospherethrough both physical processes, such as CO2 gas dissolving into thewater, and biological processes, such as the growth, death and decay ofplankton. Although most of this surface carbon cycles rapidly,some of it can also be transferred by sinking to the deep ocean poolwhere it can be stored for a much longer time.
Atmosphere: Theatmosphere contains approximately 750 PgC, most ofwhich is in the form of CO2, with much smaller amounts of methane (CH4and various other compounds). Although this is considerably lesscarbon than that contained in the oceans or crust, carbon in theatmosphere is of vital importance because of its influence on thegreenhouse effect and climate. The relatively small size of theatmospheric C pool also makes it more sensitive to disruptions causedby and increase in sources or sinks of C from the Earth’s otherpools. In fact, the present-day value of 750 PgC is substantiallyhigher than that which occurred before the onset of fossil fuelcombustion and deforestation. Before these activities began, theatmosphere contained approximately 560 PgC and this value is believedto be the normal upper limit for the Earth under naturalconditions. In the context of global pools and fluxes, theincrease that has occurred in the past several centuries is the resultof C fluxes to the atmosphere from the crust (fossil fuels) andterrestrial ecosystems (via deforestation and other forms of landclearing).
Terrestrial Ecosystems:Terrestrial ecosystems contain carbon in theform of plants, animals, soils and microorganisms (bacteria andfungi). Of these, plants and soils are by far the largest and,when dealing with the entire globe, the smaller pools are oftenignored. Unlike the Earth’s crust and oceans, most of the carbonin terrestrial ecosystems exists in organic forms. In thiscontext, the term “organic” refers to compounds that were produced byliving things, including leaves, wood, roots, dead plant material andthe brown organic matter in soils (which is the decomposed remains offormerly living tissues).
Plants exchange carbon with the atmosphere relatively rapidly throughphotosynthesis, in which CO2 is absorbed and converted into new planttissues, and respiration, where some fraction of the previouslycaptured CO2 is released back to the atmosphere as a product ofmetabolism. Of the various kinds of tissues produced by plants,woody stems such as those produced by trees have the greatest abilityto store large amounts of carbon. Wood is dense and trees can belarge. Collectively, the Earth’s plants store approximately 560PgC, with the wood in trees being the largest fraction.
The total amount of carbon in the world’s soils is estimated to be 1500PgC. Measuring soil carbon can be challenging, but a few basicassumptions can make estimating it much easier. First, the mostprevalent form of carbon in the soil is organic carbon derived fromdead plant materials and microorganisms. Second, as soil depthincreases the abundance of organic carbon decreases. Standardsoil measurements are typically only taken to 1m in depth. Inmost case, this captures the dominant fraction of carbon in soils,although some environments have very deep soils where this rule doesn’tapply. Most of the carbon in soils enters in the form of deadplant matter that is broken down by microorganisms during decay. The decay process also released carbon back to the atmosphere becausethe metabolism of these microorganisms eventually breaks most of theorganic matter all the way down to CO2.
CARBON FLUXES
The movement of any material from one place to another is called a fluxand we typically think of a carbon flux as a transfer of carbon fromone pool to another. Fluxes are usually expressed as a rate withunits of an amount of some substance being transferred over a certainperiod of time (e.g. g cm-2 s-1 or kg km2 yr-1). For example, theflow of water in a river can be thought of as a flux that transferswater from the land to the sea and can be measured in gallons perminute or cubic kilometers per year.
A single carbon pool can often have several fluxes both adding andremoving carbon simultaneously. For example, the atmosphere hasinflows from decomposition (CO2 released by the breakdown of organicmatter), forest fires and fossil fuel combustion and outflows fromplant growth and uptake by the oceans. The size of various fluxescan vary widely. In the previous section, we briefly discussed afew of the fluxes into and out of various global C pools. Here,we will pay more careful attention to some of the more important Cfluxes.
Photosynthesis: Duringphotosynthesis, plants use energy from sunlightto combine CO2 from the atmosphere with water from the soil to createcarbohydrates (notice that the two parts of the word, carbo- and–hydrate, signify carbon and water). In this way, CO2 is removedfrom the atmosphere and stored in the structure of plants. Virtually all of the organic matter on Earth was initially formedthrough this process. Because some plants can live to be tens,hundreds or sometimes even thousands of years old (in the case of thelongest-living trees), carbon may be stored, or sequestered, forrelatively long periods of time. When plants die, their tissuesremain for a wide range of time periods. Tissues such as leaves,which have a high quality for decomposer organisms, tend to decayquickly, while more resistant structures, such as wood can persist muchlonger. Current estimates suggest photosynthesis removes 120PgC/year from the atmosphere and about 610 PgC is stored in plants atany given time.
Plant Respiration:Plants also release CO2 back to the atmospherethrough the process of respiration (the plant equivalent ofexhaling). Respiration occurs as plant cells use carbohydrates,made during photosynthesis, for energy. Plant respirationrepresents approximately half (60 PgC/year) of the CO2 that is returnedto the atmosphere in the terrestrial portion of the carbon cycle.
Litterfall: Inaddition to the death of whole plants, livingplants also shed some portion of their leaves, roots and branches eachyear. Because all parts of the plant are made up of carbon, theloss of these parts to the ground is a transfer of carbon (a flux) fromthe plant to the soil. Dead plant material is often referred toas litter (leaf litter, branch litter, etc.) and once on the ground,all forms of litter will begin the process of decomposition.
Soil Respiration: Therelease of CO2 through respiration is not uniqueto plants, but is something all organisms do. When dead organicmatter is broken down ordecomposed (consumed by bacteria and fungi), CO2 is released into theatmosphere at an average rate of about 60 PgC/year globally. Because it can take years for a plant to decompose (or decades in thecase of large trees), carbon is temporarily stored in the organicmatter of soil.
Ocean—Atmosphere exchange:Inorganic carbon is absorbed and released atthe interface of the oceans’ surface and surrounding air, through theprocess of diffusion. It may not seem obvious that gasses can bedissolved into, or released from water, but this is what leads to theformation of bubbles that appear in a glass of water left to sit for along enough period of time. The air contained in those bubblesincludes CO2 and this same process is the first step in the uptake ofcarbon by oceans. Once in a dissolved form, CO2 goes on to reactwith water in what are known as the carbonate reactions. Theseare relatively simple chemical reactions in which H2O and CO2 join toform H2CO3 (also known as carbonic acid, the anion of which, CO3, iscalled carbonate). The formation of carbonate in seawater allowsoceans to take up and store a much larger amount of carbon than wouldbe possible if dissolved CO2 remained in that form. Carbonate isalso important to a vast number of marine organisms that use thismineral form of carbon to build shells.
Carbon is also cycled through the ocean by the biological processes ofphotosynthesis, respiration, and decomposition of aquatic plants. In contrast with terrestrial vegetation is the speed at which marineorganisms decompose. Because ocean plants don’t have large, woodytrunks that take years to breakdown, the process happens much morequickly in oceans than on land—often in a matter of days. Forthis reason, very little carbon is stored in the ocean throughbiological processes. The total amount of carbon uptake (92 Pg C)and carbon loss (90 PgC) from the ocean is dependent on the balance oforganic and inorganic processes.
Fossil fuel combustion andland cover change: The carbon fluxesdiscussed thus far involve natural processes that have helped regulatethe carbon cycle and atmospheric CO2 levels for millions ofyears. However, the modern-day carbon cycle also includes severalimportant fluxes that stem from human activities. The mostimportant of these is combustion of fossil fuels: coal, oil and naturalgas. These materials contain carbon that was captured by livingorganisms over periods of millions of years and has been stored invarious places within the Earth's crust (see accompanying textbox). However, since the onset of the industrial revolution,these fuels have been mined and combusted at increasing rates and haveserved as a primary source of the energy that drives modern industrialhuman civilization. Because the main byproduct of fossil fuelcombustion is CO2, these activities can be viewed in geological termsas a new and relatively rapid flux to the atmosphere of large amountsof carbon. At present, fossil fuel combustion represents a fluxto the atmosphere of approximately 6-8 PgC/year.
Another human activity that has caused a flux of carbon to theatmosphere is land cover change, largely in the form ofdeforestation. With the expansion of the human population andgrowth of human settlements, a considerable amount of the Earth's landsurface has been converted from native ecosystems to farms and urbanareas. Native forests in many areas have been cleared for timberor burned for conversion to farms and grasslands. Because forestsand other native ecosystems generally contain more carbon (in bothplant tissues and soils) than the cover types they have been replacedwith, these changes have resulted in a net flux to the atmosphere ofabout 1.5 PgC/year. In some areas, regrowth of forests from pastland clearing activities can represent a sink of carbon (as in the caseof forest growth following farm abandonment in eastern North America),but the net effect of all human-induced land cover conversions globallyrepresents a source to the atmosphere.
Geological Processes:Geological processes represent an importantcontrol on the Earth's carbon cycle over time scales of hundreds ofmillions of years. A thorough discussion of the geological carboncycle is beyond the scope of this introduction, but the processesinvolved include the formation of sedimentary rocks and their recyclingvia plate tectonics, weathering and volcanic eruptions.
To take a slightly closer look, rocks on land are broken down by theatmosphere, rain, and groundwater into small particles and dissolvedmaterials, a process known as weathering. These materials arecombined with plant and soil particles that result from decompositionand surface erosion and are later carried to the ocean where the largerparticles are deposited near shore. Slowly, these sedimentsaccumulate, burying older sediments below. The layering ofsediment causes pressure to build and eventually becomes so great thatdeeper sediments are turned into rock, such as shale. Within theocean water itself, dissolved materials mix with seawater and are usedby marine life to make calcium carbonate (CaCO3) skeletons andshells. When these organisms die, their skeletons and shells sinkto the bottom of the ocean. In shallow waters (less than 4km) thecarbonate collects and eventually forms another type of sedimentaryrock called limestone.
Collectively, these processes convert carbon that was initiallycontained in living organisms into sedimentary rocks within the Earth'scrust. Once there, these materials continue to be moved andtransformed through the process of plate tectonics, uplift of rockscontained in the lighter plates and melting of rocks in the heavierplates as they are pushed deep under the surface. These meltedmaterials can eventually result in emission of gaseous carbon back tothe atmosphere through volcanic eruptions, thereby completing thecycle. Although the recycling of carbon through sedimentary rocksis vital to our planet's long-term ability to sustain life, thegeological cycle moves so slowly that these fluxes are small on anannual basis and have little effect on a human time-scale.
In the earlier days, the conventional names for organic compounds were mainly derived from the source of occurrence & their properties. However, organic chemists realized the need for a systematic naming for organic compounds since a large number of organic compounds are synthesized in due course. This leads to setting up a system of nomenclature by 'International Union of Pure and Applied Chemistry, IUPAC'.
The IUPAC system of nomenclature is a set of logical rules framed which are mainly aimed at giving an unambiguous name to an organic compound. By using this system, it is possible to give a systematic IUPAC name to an organic compound just by looking at its structure and it is also possible to write the structure of organic compound by following the IUPAC name for that compound.
On this page, I have given a logical introduction to IUPAC nomenclature. A concise and unified approach is followed to help in giving IUPAC names to almost all types of compounds. This is not an exhaustive reference to IUPAC nomenclature. However this is more than suffice to all the students at various levels of their learning curve.
SYSTEMATIC IUPAC NAME FORMAT
The systematic IUPAC name of an organic compound consists of four parts.
- Prefix(es) and
The suffix is again divided into primary and secondary. Therefore, the complete systematic IUPAC name can be represented as:
* The 'word root' and '1o suffix' together is known as base name.
* The Prefix(es), infix and 2o suffix may or may not be required always.
1) Root word:
The Word root of IUPAC name indicates the number of carbon atoms in the longest possible continuous carbon chain also known as parent chain chosen by a set of rules. The word roots used for different length of carbon chain (upto 20) are shown below.
| Number of carbon atoms in the parent chain | Root word |
| 1 | Meth |
| 2 | Eth |
| 3 | Prop |
| 4 | But |
| 5 | Pent |
| 6 | Hex |
| 7 | Hept |
| 8 | Oct |
| 9 | Non |
| 10 | Dec |
| 11 | Undec |
| 12 | Dodec |
| 13 | Tridec |
| 14 | Tetradec |
| 15 | Pentadec |
| 16 | Hexadec |
| 17 | Heptadec |
| 18 | Octadec |
| 19 | Nonadec |
| 20 | Icos |
2) Suffix:
It is again divided into two types.
- Primary suffix and
- Secondary suffix
i) Primary suffix:
It is used to indicate the degree of saturation or unsaturation in the main chain. It is added immediately after the word root of IUPAC name.
| Type of carbon chain | Primary suffix |
| Saturated (all C-C bonds) | -ane |
| Unsaturated: one C=C | -ene |
| Unsaturated: two C=C | -diene |
| Unsaturated: one C≡C | -yne |
| Unsaturated: two C≡C | -diyne |
| Unsaturated: one C=C & one C≡C | -enyne |
ii) Secondary suffix:
It is used to indicate the main functional group in the organic compound and is added immediately after the 1o suffix in the IUPAC name.
Note: If there are two or more functional groups in a compound, the functional group with higher priority is to be selected as main functional group, which must be indicated by a secondary suffix. The remaining functional groups with lower priority are treated as substituents and are indicated by prefixes.
The suffixes as well as prefixes used for some important functional groups are shown in the following table in the decreasing order of their priority.
Also note that different suffix is used when carbon atom of the functional group is not part of the main chain.
| Name of Functional group | Representation | Suffix When carbon of the functional group is part of the parent chain | Suffix When carbon of the functional group is NOT part of the parent chain | Prefix |
| carboxylic acid | -COOH | -oic acid | -carboxylic acid | carboxy- |
| Acid anhydride | -oic anyhydride | -carboxylic anhydride | - | |
| Ester | -COOR | alkyl -oate | alkyl -carboxylate | alkoxycarbonyl- |
| Acid halide | -COX | -oyl halide | -carbonyl halide | halocarbonyl- |
| Acid amide | -CONH2 | -amide | -carboxamide | carbamoyl- |
| Nitrile | -CN | -nitrile | -carbonitrile | cyano- |
| Aldehyde | -CHO | -al | -carbaldehyde | oxo- |
| Ketone | -CO- | -one | - | oxo- |
| Alcohol | -OH | -ol | - | hydroxy |
| Thiol | -SH | -thiol | - | mercapto |
| Amine | -NH2 | -amine | - | amino- |
| Imine | =NH | -imine | - | imino- |
| Alkene | C=C | -ene | - | - |
| Alkyne | C≡C | -yne | - | - |
Note: This is not the complete reference.
3) Prefix:
The prefix is used to indicate the side chains, substituents and low priority functional groups (which are considered as substituents). The prefix may precede the word root or the infix of IUPAC name.
The prefixes used for some common side chains and substituents are shown below. (the prefixes for functional groups are already given)
| Side chain or Substituent | Prefix |
| -CH3 | methyl- |
| -CH2CH3 (or) -C2H5 | ethyl- |
| -CH2CH2CH3 | propyl- |
| isopropyl- | |
| -CH2CH2CH2CH3 | butyl |
| sec-butyl (or) (1-methyl)propyl | |
| isobutyl (or) (2-methyl)propyl | |
| tert-butyl (or) (1,1-dimethyl)ethyl | |
| -X | halo- |
| -OR | alkoxy- |
| -NO2 | -nitro |
Remember that the alkyl groups along with halo, nitro and alkoxy have the same preference. They have lower priority than double and triple bonds.
3) Infix:
The infixes, like cyclo, spiro, bicyclo, are added between the prefix(es) and root word in the IUPAC name to indicate the nature of parent chain.
* The 'Cyclo' infix is used to indicate the cyclic nature of the parent chain.
* The 'Spiro' infix is used to indicate the spiro compound.
* The 'Bicyclo' infix is used to indicate the bicyclic nature of the parent chain.
The infixes are some times called as primary prefixes.
STEPS INVOLVED IN WRITING IUPAC NAME
1) The first step in giving IUPAC name to an organic compound is to select the parent chain and assign a word root.
2) Next, the appropriate primary suffix(es) must be added to the root word to indicate the saturation or unsaturation.
3) If the molecule contains functional group or groups, a secondary suffix must be added to indicate the main functional group. This is optional and not necessary if the molecule contains no functional group.
4) Prefix the root word with the infix 'cyclo' if the parent chain is cyclic; or with the infix 'spiro' if it is a spiro compound; or with the infix 'bicyclo' if the compound is bicyclic.
5) Finally add prefix(es) to the IUPAC name, if there are side chains or substituents on the parent chain.
E.g. The IUPAC name of the following compound (3-methylbutan-2-ol) is arrived in steps mentioned below.
| Step-1 | How many carbons are there in the parent chain? | 4 | Root word = 'but' |
| Step-2 | Saturated or Unsaturated? | Saturated | 1osuffix = 'ane' |
| Step-3 | Is there any functional group? | Yes. There is an alcohol group on 2nd carbon. | 2osuffix = '2-ol' |
| Step-4 | Are there any side chains or substituents? | Yes. There is a methyl group on 3rd carbon. | 2oprefix = '3-methyl' |
Now add them to makeup the IUPAC name of the compound.
You will learn how to select a parent chain?; how to number the carbon atoms and give the locants to the functional groups, side chains ? etc., in the following section.
RULES OF IUPAC NOMENCLATURE
The following IUPAC nomenclature rules are helpful in assigning the systematic IUPAC name of an organic compound.
1) The selection of parent chain:
The first step in naming an organic compound is to select the parent chain and give the root word based on the number of carbon atoms in it.
The parent chain in an organic molecule is the longest continuous carbon chain containing as many functional groups, double bonds, triple bonds, side chains and substituents as possible.
Examples:
i) In the following molecule, the longest chain has 6 carbons. Hence the word root is 'hex-'. Note that the parent chain may not be straight.
ii) The root word for the following molecule is 'hept-' since the longest chain contains 7 carbons.
Do not come under the impression that the ethyl groups (-C2H5) are side chains and the longest chain contains 5 carbons.
The shaded part shows the longest chain that contains 7 carbons. Also look at the alternate way of writing this molecule in which the ethyl groups are expanded to -CH2CH3.
iii) In the following molecule, there are three chains of equal length (7 carbons).
However, the chain with more number of substituents (that with 3 substituents as shown in the following diagram) is to be taken as the parent chain. Thus 'hept' appears as word root in the IUPAC name of this compound.
iv) The double bonds and triple bonds have more priority than the alkyl side chains and some other substituents like halo, nitro, alkoxy etc. Hence, whenever there are two or more chains with equal number of carbons, the chain that contains double or triple bond is to be selected as the parent chain irrespective of other chain containing more number of substituents.
There are two chains with 6 carbons. But the chain with the a double bond as shown in the diagram (II) is to be selected as the parent chain.
Note: The double bond has more priority than the triple bond.
v) However, the longest chain must be selected as parent chain irrespective of whether it contains multiple bonds or not.
E.g. In the following molecule, the longest chain (shaded) contains no double bond. It is to be selected as parent chain since it contains more carbons (7) than that containing double bond (only 6 carbons).
vi) The chain with main functional group must be selected as parent chain even though it contains less number of carbons than any other chain without the main functional group.
The functional group overrides all of above rules since it has more priority than the double bonds, triple bonds, side chains and other substituents.
Remember that the functional group is king.
E.g. The chain (shaded) with 6 carbons that includes the -OH functional group is to be selected as parent chain irrespective of presence of another chain with 7 carbons that contains no functional group.
There are other situations which will decide the parent chain. These will be dealt at appropriate sections.
2) Numbering the parent chain:
i) The positions of double bonds or triple bonds or substituents or side chains or functional groups on the parent chain are to be indicated by appropriate numbers (or locants). The locants are assigned to them by numbering carbon atoms in the parent chain.
Even though two different series of locants are possible by numbering the carbon chain from either sides, the correct series is chosen by following the rule of first point of difference as stated below.
Note: In iupac nomenclature, the number which indicates the position of the substituent is called 'locant'.
The rule of first point of difference:
When series of locants containing the same number of terms are compared term by term, that series which contains the lowest number on the occasion of the first difference is preferred.
For example, in the following molecule, the numbering can be done from either side of the chain to get two sets of locants. However the 2,7,8 is chosen since it has lowest number i.e., 2 on the first occasion of difference when compared with the other set: 3,4,9.
Actually the so called “Least Sum Rule” is the special case of above “Rule of First point of Difference”. Though looking simple, the least sum rule is valid only to chains with two substituents, a special case. However use of Least sum rule is not advisable when there are more than two substituents since it may violate the actual rule of first point of difference.
Therefore, while deciding the positions, we should always use 'the rule of first point of difference' only.
ii) If two or more side chains are at equivalent positions, the one to be assigned the lower number is that cited first in the name.
In case of simple radicals, the group to be cited first in the name is decided by the alphabetical order of the first letter in case of simple radicals. While choosing the alphabetical order, the prefixes like di, tri, tetra must not be taken into account.
In the following molecule, 4-ethyl-5-methyloctane, both methyl and ethyl groups are at equivalent positions. However the ethyl group comes first in the alphabetical order. Therefore it is to be written first in the name and to be given the lowest number.
Note: The groups: sec-butyl and tert-butyl are alphabetized under 'b'. However the Isobutyl and Isopropyl groups are alphabetized under 'i' and not under 'b' or 'p'.
iii) However, if two or more groups are not at equivalent positions, the group that comes first alphabetically may not get the least number.
E.g. In the following molecule, 5-ethyl-2-methylheptane, the methyl and ethyl groups are not at equivalent positions. The methyl group is given the least number according to the rule of first point of difference.
But note that the ethyl group is written first in the name.
iv) The multiple bonds (double or triple bonds) have higher priority over alkyl or halo or nitro or alkoxy groups, and hence should be given lower numbers.
E.g. In the following hydrocarbon, 6-methylhept-3-ene, the double bond is given the lower number and is indicated by the primary suffix 3-ene. The position of methyl group is indicated by locant, 6.
v) The double bond is preferred over the triple bond since it is to be cited first in the name.
Therefore the double bond is to be given the lower number whenever both double bond and triple bond are at equivalent positions on the parent chain.
E.g. In the following hydrocarbon, hept-2-en-5-yne, both the double and triple bonds are at equivalent positions. But the position of double bond is shown by 2-ene. The counting of carbons is done from the left hand side of the molecule.
vi) However, if the double and triple bonds are not at equivalent positions, then the positions are decided by the rule of first point of difference.
E.g. In the following hydrocarbon, hept-4-en-2-yne, the double and triple bonds are not at equivalent positions. The triple bond gets the lower number.
Again note that the 4-ene is written first.
vii) Nevertheless, the main functional group must be given the least number even though it violates the rule of first point of difference. It has more priority over multiple bonds also.
For example, in the following organic molecule, 6-methyloct-7-en-4-ol, the -OH group gets lower number (i.e., 4) by numbering the carbons from right to left.
3) Grammar to be followed in writing the IUPAC name:
i) The IUPAC name must be written as one word. However, there are exceptions.
ii) The numbers are separated by commas.
iii) The numbers and letters are separated by hyphens.
iv) If there are two or more same type of simple substituents they should be prefixed by di, tri, tetra, penta etc.
E.g. The number of methyl groups are indicated by di and tri in the following cases.
v) If the side chains themselves contain terms like di, tri, tetra etc., the multiplying prefixes like bis, tris, tetrakis etc., should be used.
E.g. The two 1,2-dimethylpropyl groups are indicated by the prefix 'bis' as shown below.
vi) If two or more side chains of different nature are present, they are cited in alphabetical order.
* In case of simple radicals, they are alphabetized based on the first letter in the name of simple radical without multiplying prefixes.
E.g. In the following molecule, the ethyl group is written first since the letter 'e' precedes the letter 'm' of methyl in the alphabetical order. We should not compare 'e' in the word 'ethyl' and 'd' in the word 'dimethyl'
* However the name of a complex radical is considered to begin with the first letter of its complete name.
E.g. In the following case, “dimethylbutyl” is considered as a complete single substituent and is alphabetized under 'd'.
IUPAC Nomenclature of cyclic compounds
i) The IUPAC name of an alicyclic compound is prefixed with 'cyclo'.
E.g.
ii) Cycles are seniors to acyclics.
Hence when cyclic nucleus is attached to the non cyclic chain, it is always named as the derivative of the cyclic hydrocarbon irrespective of the length of the non cyclic chain. This is a very new IUPAC recommendation.
However, according to the 1979 convention: “a hydrocarbon containing a small cyclic nucleus attached to a long chain is generally named as a derivative of the acyclic hydrocarbon; and a hydrocarbon containing a small group attached to a large cyclic nucleus is generally named as a derivative of the cyclic hydrocarbon.” Most of the textbooks and teachers still follow this convention.
E.g. In the following examples, the old IUPAC system suggests different name when the acyclic chain contains more number of carbons than in cyclic system.
iii) When two non-aromatic rings (alicyclic) are connected to each other, the compound is considered as the derivative of larger ring. The root word is derived from the larger ring. Whereas the smaller ring is indicated by the prefix.
E.g. The following compound is considered as the derivative of cyclohexane. The smaller ring is indicated by the prefix: cyclopentyl.
iv) However if two alicyclic rings of same size are connected to each other, they are named as x,x'-bi(cycloalkyl). Where x and x' indicate the locants given to carbons through which the rings are connected. The x refers to the locant of carbon in first ring and x' represents the locant of carbon in second ring.
E.g. The following compound is named as 1,1'-bi(cyclopentyl) since there are two cyclopentane rings are connected to each other through their 1 and 1' carbons.
Primary Carbon Sn2
E.g. In the following compound two cyclopentane rings are attached to each other. Hence the name is 1.1'-bi(cyclopentyl)
v) The aromatic rings have more preference over the non-aromatic rings, when the sizes of both the rings are same.
E.g. The word root is benzene in the following compound.
However the larger ring has more priority irrespective of its nature (whether it is aromatic or not).
E.g. In the phenylcycloheptane, the non-aromatic ring, cycloheptane is larger. Hence this compound is named as the derivative of cycloheptane.
vi) Nevertheless, the functional group is always the king. It will decide the root word of the IUPAC name when present in the compound.
E.g. In the first compound as shown below, the acyclic chain is taken as parent chain since it has the -OH functional group on it. The cyclopentane part is considered as substituent.
In the second compound also the benzene ring is considered as substituent since it contains no functional group.
IUPAC name of Compounds with multi functional groups
Whenever there are more than one functions group, the main functional group is indicated by the 2o suffix in the IUPAC name, whereas the remaining functional groups are considered as substituents and are indicated by the appropriate prefixes.
E.g. In the following organic compound, 5-hydroxyhexanoic acid, both -OH and -COOH groups are the functional groups. But the -COOH group has more priority than the -OH group. Hence it is considered as the main functional group and indicated by secondary suffix, 'oic acid'. Whereas the -OH group is considered as substituent and is indicated by the prefix, 'hydroxy'.
IUPAC nomenclature of Spiro compounds
The spiro compounds contain two cyclic rings that share one common carbon atom, which is called as the spiroatom.
Secondary Carbons
The IUPAC name of spiro compound has the infix 'spiro' followed by square brackets inside of which the number of atoms in the smaller ring followed by the number of atoms in the larger ring, excluding the spiroatom itself, are shown. These numbers are separated by a period (dot).
The word root of the compound is based on the total number of skeletal carbons in the two cycles including the spiroatom. Do not include the carbons of side chains and substituents over the rings while counting this number.
E.g. In the following spiro compound, there is one carbon atom common to 5 membered and 6 membered rings. The IUPAC name is spiro[4.5]decane. Notice that the spirocarbon is not taken into account while giving the numbers in the square bracket.
The numbering is done starting from skeletal carbon of small ring and continued until the spiro carbon. Then the skeletal carbons in the larger ring are numbered.
E.g. In the following spiro compound the methyl group has got the locant, 7. It is because the numbering of the spiro skeleton is done first and it is not necessary that the methyl group should get the least number always.
IUPAC nomenclature of Fused bicyclic compounds
The bicyclo compounds contain two fused rings with two connecting common carbon atoms known as bridge head carbons. The carbon chain or covalent bond connecting these bridge heads is considered as a bridge. There are three bridges in a simple bicyclic compound.
The IUPAC name of bicyclic compound has the infix 'bicyclo' followed by square brackets showing the numbers separated by periods (dots). They indicate the number of atoms in the bridges. While counting the number of atoms in the bridge, the bridge head carbons are not counted. These numbers are arranged in the decreasing order i.e., from larger bridge to smaller one.
The root word indicates the total number of skeletal carbon atoms in the two rings. Do not include the carbons in side chains or substituents over the rings while arriving at the word root of IUPAC name.
Primary Secondary Tertiary Quaternary Carbon
E.g. In the following bicyclo compound, there are three bridges with 2, 2 and 1 carbon atoms connecting the two bridge head carbons. Hence the name is bicyclo[2.2.1]heptane.
The numbering is done starting from one of the bridge head carbon and continued through the longest bridge until another bridge is reached. Then the skeletal carbons of next longer bridge are numbered. This process is continued until the shortest bridge in finally numbered.
E.g. In the following bicyclo compound, the methyl group is is considered to be at 7th position.
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