UNIT 2: ALKANES, ALKENES AND ALKYNES
PART 2: ALKENES & ALKYNES
A “functional group” is a segment of an organic molecule that has unique physical and chemical properties. The
simplest type of functional group is a pi bond, or a pair of delocalized electrons. Like alkanes, alkenes and alkynes
contain only hydrogen and carbon. However, alkenes and alkynes are unsaturated, or do not contain the maximum
number of hydrogens possible due to the presence of double (alkene) and triple (alkyne) bonds.
1. MOLECULAR FORMULA AND NAMING OF ALKENES:
a. Each molecule below is an alkene. Write the molecular formula for each molecule in the space provided.
__________________
propene (propylene)
_________________
2-butene (but-2-ene)
(2-butylene)
________________________
2-pentene
pent-2-ene
b. Study each molecular formula that you wrote. Write a generic formula for alkenes.
c. Use the formula you found above to try to predict the molecular formula of an alkene with 7 carbon atoms:
d. The name for each of the molecules is given below the diagrams. Notice that the “ene” suffix replaces the “ane” that
was used for alkanes. Why do you think the number 2 is included in the name for 2-pentene and 2-butene? Why isn’t
included for propene?
NOTE: The names “2-pentene” and “pent-2-ene” are both IUPAC and commonly used today. Ethylene and butylene
are named in the “common naming system”, but are not IUPAC names. You need to be familiar with this form, but the
correct IUPAC names are propene and ethene.
2. NAMING ALKENES:
The steps for naming alkenes are given below.
 1. Find the longest continuous carbon chain that contains the double bond, and number from the end nearer the
double bond.
 2. Write the alkane name corresponding to the number of carbons, changing the –ane to –ene.
 3. Identify by number the lower-numbered carbon atom containing the double bond, followed by a hyphen
directly before the alkene name.
 4. If substituent groups are present, they should precede the name of the alkene in the usual way.
CH3
H3C
H3C
EXAMPLE: Name the following molecule:
CH3
CH3
H3C
5
6
4
3
H3C
CH3
2
1
STEPS 1 & 2 : hexene
STEP 3: 2-hexene
hex-2-ene
STEP 4: 4-ethyl-5-methyl-2-hexene
4-ethyl-5-methyl-hex-2-ene
PRACTICE: Name the following molecules:
CH3
CH3
CH3
H3C
CH3
H3C
CH2
3. NAMING POLYUNSATURATED ALKENES:
A polyunsaturated alkene has more than one double bond. The molecule below is a diene. To name a diene;
 List the lower numbered carbons containing the double bonds, separated by a comma.
 After the alkane name, -di- is used to signify 2 double bonds, followed by –ene.
4. MOLECULAR FORMULAS AND NAMING FOR ALKYNES:
CnH2n-2
Alkynes are unsaturated hydrocarbons that contain a triple bond. To name alkynes, follow the same process you used
for naming alkenes except substitute the suffix –yne.
TRY: Name the molecules below. Use the prefix chloro- to indicate the presence of a chlorine atom. Then check your
answers:
H3C
CH3
H3C
CH3
c. H3C
Cl
H3C
CH
b.
CH
a.
Answers: 1-butyne;
but-1-yne;
4-methyl-2-pentyne;
4-methyl-pent-2-yne;
3-chloro-4-methyl-1-pentyne
3-chloro-4-methyl-pent-1-yne
5. CYCLIC ALKANES, ALKENES & ALKYNES
Cycloalkanes, cycloalkenes and cycloalkynes have ring structures. The prefix “cyclo” is used in the name. Numbering
begins at the multiple bond and moves in the direction toward the nearest branch, the goal being to assign the lowest
numbers possible to the branch. All other naming rules are followed.
EXAMPLES:
3
1
6
5
CH3
CH3
2
CH3
4
4
3
5
4
3
2
2
1
Br
5-bromo-3-ethylcyclohexene
Cl
CH3
5
CH3
6
1
CH3
3-ethyl-5-methylcyclohexyne
1-chloro-2,3,4-trimethylcyclopentane
1. Name the following molecules:
CH3
CH3
H3C
H3C
CH3
2. Write the molecular formula for each molecule. How does it compare with a noncyclic molecule
containing the same number of carbon atoms?
COMMON BRANCH NAMES: ISO-, TERT- & SEC-
Number of C Atoms
in group
3
4
Line Drawing
Name
isopropyl
Three carbon branch coming off
of a chain; attached at the middle
carbon
isobutyl
Four carbon branch coming off of
a chain, attached at the second
carbon
Same as iso-
4
4
5
sec-butyl
tert-butyl
Four carbons coming off of a
chain, in a “methyl propyl”
arrangement
isopentyl
5-carbon branch coming off of a
chain, attached at the second
carbon
Same as iso-
5
sec-pentyl
Examples:
4-isopropyl nonane
secbutyl cyclopentane
CH3
CH3
4-tertbutyl-cyclohexene
TRY: 3-tertbutyl -4-propyl-2-octene
LOTS OF PRACTICE & CONDENSED FORMULAS: Complete the charts below. Leave the condensed formula column
blank. We will work on it together.:
NAME
STRUCTURAL DIAGRAM
CONDENSED FORMULA
4,4 dimethyl-2-pentene
3-methyl-4-propyl-1heptene
3-isopropyl cyclo-octyne
4-ethyl-2-hexyne
H3C
CH3
H3C
CH3
H3C
CH3
CH3
H3C
H3C
Br
CH3
ALKENES/ALKYNES – HOMEWORK 1
NAME:
DATE:
1. Name each of the alkenes/alkynes below.
H3C
CH3
H3C
CH3
H3C
CH3
a.
H3C
CH3
4,4 dimethyl 2 hexene
H3C
b.
4-secbutyl-2-octyne
CH3
H3C
CH3
2,4-dimethyl-2-pentene
H3C
2-bromo-3-methyl-2-pentene
H3C
e.
CH3
CH3
f.
4-ethyl-2-fluoro(2,4) hexadiene
CH2CHCH2CH(CH3)2
b. (CH3)3CCCCH2CH2CH2Cl
H3C
CH3
H3C
H2C
CH3
CH3
4-methylpent-1-ene
CH3
4-propyl cyclohexyne
3. Draw and name each molecule below based on its condensed formula:
a.
CH3
F
CH3
d.
c.
Br
Cl
7-chloro-2,2-dimethylhept-3-yne
4. Draw the molecules listed below:
a. 2,3 dichloro-3-heptene
b. 3-hexene
Cl
H3C
H3C
Cl
CH3
CH3
5. Complete the chart:
CONDENSED FORMULA
STRUCTURAL FORMULA
NAME
2-methyl-3-hexene
H3C
CH3CH2CHCHCH(CH3)2
CH3
H3C
(3E)-2-methylhex-3-ene
Cl
CHC(CH2)5CH2Cl
HC
8-chlorooct-1-yne
CH3
(CH3)3CCHCH(CH2)2CHCH2
7,7-dimethyl-(1,5) octadiene
H3C
CH3
CH2