Alkenes

 

Alkenes have the general formula CnH2n. The first two members of the series have the common names ethylene and propylene.

Ethylene: CH2=CH2 Propylene: CH2=CHCH3

Ethylene has been accepted as an official IUPAC name.

 

  1. (a) Write the structural formulas for tetrafluoroethylene which is the building block for production of Teflon.

    2.  

     

     

    (b) Write the structural formula for trichloroethylene, a solvent used in dry cleaning.

     

     

     

    (c) What is the name given to CH2=CCl2?

     

     

     

     

     

    (a) CF2=CF2

    (b) CHCl=CCl2

    (c) 1,1-Dichloroethylene to distinguish it from

    CHCl=CHCl

     

     

     

    IUPAC Names for Alkenes

    Select the longest carbon chain containing the double bond and assign this a parent name by changing the "ane" of the corresponding alkane to "ene".

     

     

    CH3CH2CH3

    CH3CH=CH2

    IUPAC:

    Propane

    Propene

    Common Names:

    ---

    Propylene
     

    (CH3)2CHCH3

    (CH3)2C=CH2

    IUPAC:

    2-Methylpropane

    2-Methylpropene

    Common Names:

    Isobutane

    Isobutylene


     

  2. Consider the following carbon skeleton:



    3.  

    (a) How many carbons are in the longest carbon chain?

     

     

     

    (a)

     

     

    (b) How many carbons are in the longest carbon chain containing the double bond? What is the parent name for the alkene?

     

     

    (b)

     

     

     

    Number the parent carbon chain from the end nearer the double bond; indicate the position of the double bond by the number of the first doubly-bonded carbon encountered.

  3. Returning to the structural skeleton from question 2.

    4.  

    (a) What is the position of the double bond? What is the name of the parent alkene?

     

     

     

     

    (b) Give the complete name of the alkene with the above carbon skeleton.

     

     

     

    4-Butyl-4, 6-dimethyl-2-nonene

     

     

     

     

     

     

  4. Name the following compound.

 

 

 

 

 

3-Chloro-5-ethyl-2-methyl-2-heptene

 

 

 

5. Name the following compound:

 

 

 

 

5-Ethyl-3-(3-methylbutyl)-1-octene

or

5-Ethyl-3-isopentyl-1-octene

 

 

 

 

Cis/Trans or Geometric Isomers

The stereochemical designation for 1,2-disubstituted double bonds is straightforward. In printed text the prefixes cis- and trans- are italicized and not capitalized.

 

Cyclic compounds can also exhibit cis/trans isomerism.

 

 

 

Various representations of trans-1,4-dimethylcyclohexane

 

 

6. Name the following compound.

 

 

 

 

trans-5-methyl-3-heptene

 

 

 

E and Z Stereochemical Designations

Because cis and trans designation can become confusing in cases more complicated than simple disubstituted alkenes, the stereochemical descriptors E and Z are used.

 

The various groups on each carbon of the carbon-carbon double bond are assigned a priority. The configuration in which the two groups of higher priority (the two 1’s) are on the same side is give the descriptor (Z)- (from the German zusammen, together); the configuration in which the higher priority groups are on the opposite sides is given the descriptor (E)- (from the German entgegen, opposite). To utilize this system, often referred to as the Cahn, Ingold, Prelog system, one must become familiar with the priority rules.

 

Sequence Rule 1

Groups are arranged in priority in order of decreasing atomic number of the groups attached to a carbon.

In the above example the left hand carbon of the double bond (carbon 2) has attached to it a carbon (At. No. 12) and a hydrogen (At. No. 1). The former is of higher priority. The right hand carbon of the double bond has attached to it a carbon (At. No. 12) and a chlorine (At. No. 17). The latter is of higher priority. The two higher priority groups are on the same side and are therefore Z.

A hint from Bruice to remember this: "Zee groups are Zee Zame Zide."

 

7. Give the (E) or (Z) stereochemical designation of the following compound.

 

 

 

 

 

 

 

 

 

 

 

 

8. Give another name for cis-3-pentene.

 

 

(Z)-3-pentene

 

 

 

 

 

Sequence Rule 2

If atoms attached directly to a carbon are the same, the one substituted with atoms of higher atomic number takes precedence.

 

 

In the above examples, look at the two groups attached to the green carbons. In both instances the atom directly attached is a carbon, so no decision of priority can be made. In the ethyl case the first carbon is further attached to two hydrogens (At. No. 1) and a carbon (At. No. 12). In the bromomethyl case, the first carbon is further attached to two hydrogens (At. No. 1) and a bromine (At. No. 35). The bromomethyl group, therefore, is of higher priority.

If the second array of atoms permits no choice, the third atoms out are examined, etc.

 




9. Arrange the following groups according to priority (from highest to lowest):

n-butyl, isobutyl, sec-butyl, tert-butyl

 

 

 

 

 

 

  

 

Sequence Rule 3

 

If an atom is attached to another atom by multiple bonds of the p-p p (double or triple in the case of carbon) both atoms are regarded as being replicated.

 

 

10. Which group has higher priority, isopropyl or CH2=CH- ? To answer this question first write out the two groups as expanded structures replacing any multiple bonds with "replicated" atoms. You will immediately notice that the attached atoms are the same, a carbon. In the isopropyl case what are the second atoms out? In the vinyl case what are the second atoms out? In the isopropyl case, what are the third atoms out? In the vinyl case what are the third atoms out? Which group has highest priority?

 

 

 

 

In the isopropyl case what are the second atoms out?

2 C’s and 1 H

In the vinyl case what are the second atoms out?

2 C’s and 1H

In the isopropyl case, what are the third atoms out?

6 H’s

In the vinyl case what are the third atoms out?

1 C and 1 H

Which group has highest priority? Vinyl

 

 

 

 

 

Sequence Rule 4

An isotope of higher mass number takes precedence, i.e., D has higher priority than H.

Isotopes can be represented by using a superscript before the symbol of the element to represent the mass number of the isotope.

Carbon-13: 13C

Deuterium: 2H or D

Tritium: 3H or T

 

 

 

 

11. Draw the (E) isomer of [1-2H,3-13C]-2-methylpropene.

 

(E)-[1-2H,3-13C]-2-methylpropene

 

 

 

 

 

12. If both alkenes and cycloalkenes have the general formula CnH2n, then cycloalkenes must have the general formula _____________.

 

CnH2n-2

 

 

 

 

 

In names of cycloalkenes, the carbons involved in the double bond are assigned numbers 1 and 2. The direction of numbering is chosen so as to give the lower possible numbers designations to substitutents on the occasion of the first difference. Since the double bond will always be a position 1, it is not necessary to specify this in the name.

It is also not necessary to specify stereochemistry about the double bond in cycloheptene and smaller rings; hydrogens or substitutents in the 1 and 2 positions are asssumed to be cis.

3-Methylcyclohexene

 

13. Name the following compound:

 

 

 

5-Bromo-1-methylcyclopentene

(Not 3-bromo-2-methylcyclopentene. The locants 1,5 are preferred over 2,3 as the former has the lower number on the occasion of the first difference. The locant for the double bond as in 5-bromo-1-methyl-1-cyclopentene is often omitted as it is determined by the rules to be 1.)

 

 

 

14. Name the following compound:

 

 

 

3,6-Dichloro-4,4,5-trimethylcyclohexene

 

 

In IUPAC nomenclature, dienes, trienes, tetraenes, etc. (generically known as polyenes) are named in the same manner as alkenes except that the ending "ene" is replaced by "adiene", "atriene", "atetraene", etc. A number must be used to indicate the position of each double bond.

CH3CH=C=CH2

CH2=CH-CH=CH2

1,2-Butadiene

1,3-Butadiene

 

 


15. Name the following compound:

 

1,4-Cyclohexadiene

 

 

 

16. There are several dienes which have important common names that are recognized by IUPAC. Isoprene is an important building block for many naturally occurring materials.

What are the systematic names for allene and isoproene?

 

 

 

 

 

 

In specifying configurations about double bonds in polyenes, the stereochemical descriptors (E) and (Z) are used with the appropriate locants.

 

 

 

 

17. a-Farnescene is an ingredient in the waxy coating of apple skin. Give the IUPAC name of this compound.

a-Farnescene

 

 

 

(3E,6E)-3,7,11-Trimethyl-1,3,6,10-dodecatetraene

 

 

 

18. Draw the structure of bicyclo[2.2.1]hepta-2,5-diene. (Note the position of the locants in the name.)

 

 

 

 

Bicyclo[2.2.1]hepta-2,5-diene

 

 

 

Alkenyl groups:

Naming of substituents containing a double bond.

Alkenyl Group

Systematic Name

Common Name

 

CH2=CH-

 

Ethenyl

 

Vinyl

CH2=CH-CH2-

2-Propenyl

Allyl

 

Vinyl chloride: CH2=CHCl

Allyl alcohol: CH2=CH-CH2OH

 

Related terms are used in a generic sense to refer to types of carbons. The sp2 carbons of a carbon carbon double bond are called vinylic carbons; sp3 carbons adjacent to a carbon-carbon double bond are called allylic carbons. Hydrogens attached to these carbons are called vinylic and allylic hydrogens, respectively.

 

 

 

19. Name the following compound: CH3OCH=CH2.

 

 

 

Methyl vinyl ether

 

 

 

20. Name the following compound.

 

 

 

 

 

4-Vinylcyclohexene or 4-Ethenylcyclohexene

 

 

 

 

If a methylene (=CH2) group is attached to a ring carbon, the compound is named as a methylene derivative of the cycloalkane.

1-tert-Butyl-4-methylenecyclohexane

 

21. Name the following compound. How many vinylic hydrogens are present in this compound? How many allylic hydrogens?

 

 

 

1-methylene-2-vinylcyclopentane

or 1-ethenyl-2-methylenecyclopentane

5 Vinylic hydrogens

3 Allylic hydrogens

 

 

 

Go to Chapter on

Stereoisomers