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1.2 Alkenes and Alkynes
 Alkenes contain one or more double bonds (General formula: CnH2n)
 Alkynes contain one or more triple bonds (General formula: CnH2n-2)
 Since alkenes and alkynes are NOT bonded to the maximum possible number of
atoms, these compounds are often referred to as unsaturated hydrocarbons
Boiling points and melting points:
Reactivity:
alkynes < alkenes < alkanes
alkynes > alkenes > alkanes
When naming these molecules, there are a few extra rules to follow:
• The main chain must include double or triple bonds at lowest position number
(indicate position #)
• alkenes end with –ene, alkynes end with -yne
• if more than one double bond exists use prefixes (diene, triene, etc)
• when numbering the main chain, double and triple bonds have priority over alkyl
groups
Name the following:
(a)
C
H
3
C
H
H
C
HC
HC
HC
3
3
(b)
C
H
H
H
H
C
H
3 C
3
2 C
2 C
2 C
C
H
C
H
3
(c)
(d)
CH3C(CH3)2CH(CH2CH3)CCH
(e)
Draw the following:
(a)
3-methylpent-2-ene
(b)
2,5,7-trimethyl-5-propyloct-3-yne
Name the following Cyclic Alkene:
Draw the following:
3,3-diethyl-2-methylcyclopentene
Stereoisomers: Cis and Trans
 same number atoms, with the double bond in the same position, but with a different 3D
geometry around that double bond
 Cis means that the two groups are on the same side of the double bond
 Trans indicates that they are on ‘opposite’ sides
ex)
cis-but-2-ene
trans-but-2-ene
Self Check
How prepared am I to start my homework? Can I …
… name and draw alkanes and alkenes
… name and draw stereoisomers for an alkene
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