Aikenes and Their Chlorinated Derivatives

In some organic molecules, one or more pairs of the carbon atoms are joined by double bonds; since each carbon atom forms a total of four bonds, there are only two additional bonds formed by such carbon atoms. The simplest hydrocarbon of this type is a colorless gas called ethem, usually known by its older name ethylene:

ethene (ethylene)

Notice that the actual planar geometry of this molecule, with bond angles of about 120° around each carbon, can be shown in the structural formula. Condensed formulas normally show the double bond: CI l2 = CH2 or HiC^CHi.

A C = C bond can be a part of a longer sequence of carbon atoms that are joined together by other single, double, or triple C--C bonds. For example, propene is a three-carbon chain with one adjacent pair of carbons joined by a double bond:

propene

The name for a hydrocarbon chain containing a C=C bond is the same as that used for the alkane of the same length, except that the -ane ending of the alkane is replaced by -ene. The molecule is numbered such that the C = C unit is part of the continuous chain and such that the C=C unit is at the lower-numbered end of the chain. Collectively, hydrocarbons containing C = C bonds are called aikenes. If there are two C=C bonds in a

propene hydrocarbon, the prefix di- is placed before the -ene ending; thus the hydrocarbon below is called J ,3-pentadiene:

H

\

H

C=

\

/

O

=C

/

\

H

1,3-pentadiene

The numbers preceding the name are those assigned to the first carbon atom that participates in each of the double bonds. The alternative numbering scheme, i.e., assigning the CH3 carbon on the right to be #1, is not used since the first double bond would then start at carbon #2 and the name would be 2,4'pentadiene; thus the first double bond would not have the lowest possible number.

In some derivatives of ethene, one or more of its hydrogen atoms have been replaced by chlorine atoms. The chloroethenes, like ethene itself, are planar molecules. The simplest example is CH2—CHCl, called chloroethene but known in the chemical industry as vinyl chloride; it is produced in huge quantities since the common plastic material polyvinyl chloride (PVC) is subsequently prepared from it.

A number is usually placed in front of the name of the substituent to indicate the specific carbon atom to which it is bonded; thus C^C—QHh is called 1,1 -dichloroethene to distinguish it from 1,2'dichloroethene, CHCl=CHCl. The molecule 1,1,2'trichkrroethene, CQ: = CHC1, is a liquid solvent that has extensive uses. Note that the prefix numbers in the compound name here are superfluous since it has no isomers and thus there is no need to distinguish one isomer from another. This compound is usually referred to by its traditional name trichloroethylene. The structural formulas of a few substitution products of ethene are:

CI

H

CI CI

\

/

\ _ /

C=

=C

C """ c

/

\

/ \

CI

H

CI CI

1,1 -dichloroethene tetrachloroethene

1,1 -dichloroethene tetrachloroethene

The liquid compound tetrachloroethene, CCl2=CCl2, is used on a large scale as the dry-cleaning solvent used commercially to remove grease spots and other stains on clothing. The prefix 1,1,2,2- is not used as part of its name since it is superfluous (no other arrangements of chlorine being possible).

Note that when all the hydrogens in a molecule have been replaced by a given atom or group, the prefix per can be used instead of the actual number; thus tetrachloroethylene is also called perchbroethylene, giving rise to its nickname "perc."

PROBLEM 2

Write structural formulas for each of the following: (a) 1,1 -dichloropropene, (b) perchloropropene, (c) 2-butene.

PROBLEM 3

Determine the correct name for each of the following: (a) CHC12CHC12, (b) CH3 CH2 CH=CH2, (c) CH2 —-CH CH=CH2.

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