Iupac Nomenclature Exercises In Organic Chemistry

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Updated on 22nd January, 2010

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IUPAC NOMENCLATURE EXERCISES IN ORGANIC CHEMISTRY Propose IUPAC names for the following compounds Note: Key with possible explanation is given at the end. C2H5

C2H 5

1)

OR

CH

CH2

H3C

CH

CH3

CH3

CH3

CH3

H3C

2)

CH3

CH

OR

CH

CH

CH

CH3

CH3

CH2

H2C

CH2 CH3

CH3

H3C

3)

CH2

CH

OR

CH

CH

CH3

CH3

CH2

H2 C

CH3

CH

CH3

CH3

4)

OR

CH

CH2

CH3 C

OR

CH2 CH

H3C CH2 H3C

CH3 CH2

CH

CH2

CH2

CH3

H3C

5)

CH2

CH

H3 C

H

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CH3

CH2

CH3

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CH3 CH CH

6)

OR

CH2 H3C

CH2 CH2

CH CH2

CH3 CH2 CH2

CH

CH2 CH2

CH3

CH2

9)

11)

13)

10)

12)

14)

16)

H

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15)

8)

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7)

M

H3C

17)

18)

19)

20)

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H3C

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22)

24)

25)

27)

29)

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23)

26)

28)

30)

KEY WITH EXPLANATION

C2H5

1)

3,4-Dimethylhexane

and not

2-Ethyl-3-methylpentane

CH3

Explanation: * This is the common mistake observed. The longest chain is NOT always expected to be arranged straight. The parent chain may be represented zigzag. The condition is the carbons in the chain must be continuous. In this case, longest chain contains six carbons continuously as shown below. 1

3 4 5

H

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2

1

2

C2H 5

CH2 -CH3

6

OR

CH3

3

4

5

6

CH3

2) 2,3,5-Trimethyl-4-propylheptane

Explanation: * If chains of equal length are competing for selection as parent chain in a saturated branched acyclic hydrocarbon, then the choice goes to the chain which has the greatest number of side chains.

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2,5-Dimethyl-4-(2-methylpropyl)heptane or

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Explanation: * In this case, The chain whose side chains have the lowest-numbered locants is taken as the main chain. * Note the two versions. Isobutyl radical is also a valid name and it can also be named as 2methylpropyl. * Following names may be used for the given unsubstituted radicals only. H3C

H3C

CH

Isopropyl

CH

Isopentyl

H3C

H3C

CH2

CH3

H3C

CH

Isobutyl

CH2

CH2

Neopentyl

H3C

C

H3C

CH2

CH3

CH3

sec-Butyl

H2C

CH

CH3

tert-Pentyl

H3C

CH2

CH3

H3C

CH3

tert-Butyl

H3C

C

H3C

H3C

2,7,8-Trimethyldecane

CH2

CH2 CH2

and not 3,4,9-Trimethyldecane even though 3+4+9 = 16 --- least sum

H

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CH

Isohexyl

CH3

4)

C

whereas 2+7+8 = 17 --- first locant is least

Explanation * When series of locants containing the same number of terms are compared term by term, that series is "lowest" which contains the lowest number on the occasion of the first difference. * Actually the so called “Least Sum Rule” is the special case of above “Rule of First Difference”. And incidentally, in most of the simple cases we still use “Least Sum Rule”. But this becomes tedious when there are more than two substituents and where the actual rule will come to the surface. (If you don’t use “least sum rule” neglect this statement.)

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4-Isobutyl-2,5-dimethylheptane

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5)

6)

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Explanation: * 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. For example, compare ‘e’ in ethyl with ‘m’ methyl, and not ‘e’ in ethyl with ‘d’ in dimethyl. BUT CONSIDER THE NEXT PROBLEM.

6-(1,2-Dimethylbutyl)-5-ethylundecane

Explanation: * The name of a complex radical is considered to begin with the first letter of its complete name. * In this case, “dimethyl” is included in the radical name (Note: “di”does not indicate two methylbutyl side chains). * Hence dimethylbutyl (as complete single substituent) is alphabetized under "d". 7)

3-Ethyl-4-methylhexane

Explanation: * If two or more side chains are in equivalent positions, the one to be assigned the lower number is that cited first in the name. * Previously, the less complex side chain is assigned the least number, which is discarded now. 8)

Explanation: * In this case, ethyl and methyl groups are not at equivalent positions. Hence according to “Rule of First Difference”, methyl is given the lowest number.

H

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4-Ethyl-3-methylheptane

9) 7-(1-methylbutyl)-9-(2-methylbutyl)pentadecane

Explanation: * In this case the names of complex radicals are composed of identical words. Hence priority is given to that radical which contains the lowest locant at the first cited point of difference in the radical.

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3-Ethyl-2,2-dimethylhexane

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10) 5,5-Bis(1,2-dimethylpropyl)nonane OR

11)

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Explanation: * Multiplying prefixes like bis-, tris-, tetrakis-, pentakis- etc., are used to indicate more than one identical side chain which contains terms like bi-, tri-, tetra- etc., in its name. * The side chain may be enclosed in parentheses or carbon atoms in the side chains may be indicated by primed numbers.

4,6-Diisopropylnonane OR 4,6-Di(propan-2-yl)nonane

Explanation: * Note the use of “propan-2-yl” for isopropyl radical. 12)

5-tert-Butyl-6-isobutyldecane

13)

H

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Explanation: * The prefixes - “tert” or “sec” are not taken into consideration for deriving alphabetical priorities. But “iso” is considered to be the part of radical name and important in deciding alphabetical priority. * In this case, tert-Butyl is alphabetized under “B” whereas isobutyl under “i”

5-tert-Butyl-6-sec-butyldecane

Explanation: * tert-Butyl group is given priority over sec-butyl group. (Why?) 14)

(3E)-7-Methyloct-3-ene

Explanation: * Double bond must be given priority (and hence lowest number) over “alkyl” groups. * Only the lower locant of the double bond ( i.e., 3) is cited as the locants of double bond differ by unity (i.e., 3,4). * It is in ‘E’(Entgegen) geometrical configuration. The groups with higher priority are on the opposite sides of double bond.

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5,5-Bis-1',2'-dimethylpropyl)nonane

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2 1

H

2

1

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* But the following is in ‘Z’(Zusammen) configuration i.e., groups with higher priorities are on the same side of double bond. 2

2

H

H

(3Z)-7-Methyloct-3-ene

1

1

Additional information: E-Z Notation: Following procedure can be adopted while arriving at the stereochemistry around the double bond and can be denoted by E or Z descriptors. (Remember, if any of the doubly bonded carbon is attached to two similar groups, then there is no need of giving E-Z descriptors.)

The priorities are assigned by following Cahn-Ingold-Prelog sequence rules. * Rank the atoms directly attached to the olefinic carbon according to their atomic number. High priority is given to the atom with higher atomic number. * In case of different isotopes of same element, then higher priority is given to the isotope with higher atomic mass. (Eg., D>H, C13>C12) * If the atoms are still identical, examine the next atoms along the chain until a “first point of difference” is found. This is done by making a list of atoms linked directly to the atom. Each list is arranged in order of decreasing atomic number. Then the higher priority is given to the list which contain atom with higher atomic number at first point of difference. Eg., Examine the lists of atoms directly linked to the highlighted carbons in the following compound, (2Z)-2-tert-Butyl-3-methylpent-2-en-1-ol.

H

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* Determine the higher priority group on each end of the double bond. If the higher priority groups are: on opposite sides of double bond: E (entgegen = opposite) on the same side of double bond: Z (zusammen = together)

O,H,H

C,H,H H3C

*CH

* CH 2 OH

2

C

C

* C

C*

H3

H,H,H

H3C

C,C,C CH3 CH3

* Multiple bonds are counted as multiples of that same atom i.e., each  - bond is treated as if it were another  - bond to that type of atom. Eg.,

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H

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* C

C

CH3

is equivalent to

C

C

C*

C

C

C

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CH3

C*

C*

CH3

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CH3

is equivalent to

M

O C

O

O

O,O,C

15)

(2E,4E)-3-Methylhexa-2,4-diene

Explanation: * It is a diene. The positions of two double bonds are indicated by 2,4. At each double bond the configurations are indicated by 2E,4E. * Starting from either side of the chain, the double bonds are getting same numbers. Hence the carbons on the main chain are numbered such that methyl group is given the least number. 16)

(4E)-4-Methylhept-4-en-2-yne

Explanation: * The triple bond is given the lowest number. * Another common mistake observed is, students name this compound as (4E)-4Methylhept-3-en-5-yne, which is wrong. Remember the ‘Rule of first point of difference’. In this case, the carbons are counted such that triple bond gets least number. However consider the next problem. 17)

Explanation: * Double bond is given the lowest number when both double and triple bonds are at equivalent positions.

H

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(2Z)-4-Methylhept-2-en-5-yne

18) (3Z)-3,4-Diethylhexa-1,3-dien-5-yne

Explanation: * When two chains of equal length are competing, then the chain with maximum number of double and triple bonds is selected as main chain. * Observe the notation (3Z). There is no need to assign E-Z descriptor for first double bond. * In this case also, double bond is given more priority while numbering the chain.

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C,C,C

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Ethenyl group(also named as vinyl group)

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Explanation: * In this case, longest chain contains eight carbons and it should be taken as the parent chain irrespective of whether it contains maximum number of unsaturations or not. * Selection of chain containing maximum number of unsaturated bonds as main chain (old IUPAC), even though there is a longer chain containing less number of double or triple bonds, is not followed now. Additional information: * The names of univalent radicals derived from unsaturated acyclic hydrocarbons have the endings “-enyl”, “-ynyl”, “-dienyl”, “-diynyl” etc., and the positions of double and triple bonds must be indicated where ever necessary. H 2C

vinyl or ethenyl

CH

allyl or prop-2-en-1-yl

CH

H2C

CH2

CH

H3C

CH

prop-1-en-1-yl

CH2

isopropenyl or prop-1-en-2-yl or 1-methylvinyl

C

H3C

HC

H

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HC

H 3C

20)

Ethynyl

C

C

CH2

C

C

Prop-2-yn-1-yl

Prop-1-yn-1-yl

3-methylidene-5-(prop-1-yn-1-yl)decane

Explanation: * The names of divalent radicals, formed by removing two hydrogens from a carbon atom of acyclic hydrocarbon end with “-idene”.

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Ethynyl group

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(4Z)-4-Ethenyl-5-ethynyloct-4-ene

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methylidene or methylene

H 2C

CH

CH2

H3C

CH

H3C

propan-1-ylidene or propylidene

propan-2-ylidene or Isopropylidene

C

H3C

C

4-ethenylideneheptane or 4-vinyledeneheptane

Note: It is a cumulene. The carbon shown is “sp” hybridized. 22)

Hexylcyclopentane or 1-Cyclopentylhexane

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Explanation: * In the first name, cyclopentane is given more priority, eventhough the straight chain contains more carbons, and taken as root word. This is according to the rule -“cycles are senior to acycles”.

H

* In the second case, hexane is taken as root word as it contains more number of carbons than cyclopentane. This type of naming was actually suggested in 1979 recommendations and can be stated as follows. “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 older textbooks follow the second convention.

23) 1-ethyl-2-methylcyclohexane

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ethenylidene or vinylidene

C

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H2C

M

Ethylidene

H3C

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24) ethenylcyclobutane

What about

?

It is (3Z)-1-ethenyl-3-ethylidenecyclopentane

27)

(1E)-prop-1-en-1-ylcyclopropane

cyclopentene

28)

29)

cyclopenta-1,3-diene

2

3

30)

10

9

(1E,3Z,5E,7Z,9Z)-cyclodeca-1,3,5,7,9-pentaene

5 6 7

H

4

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1

8

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1-ethenyl-3-ethylcyclopentane

26)

M

methylidenecyclohexane

25)