CHEM 109A
CLAS
Alkenes and Reactions of Alkenes - KEY
1. Predict the product(s) for each of the following reactions. If there is more than
one product, predict the relative yield (a.k.a. label the major product).
H
a.
Br
2-butene
H
2-butene
Br-
Br
hydrobromic acid
H
Br
H
H
b.
Cl
cyclohexene
H
Cl
Cl Cl
cyclohexene
H
H
H
c.
trans-3-methyl-pent-2-ene
H
H
H
I
I
trans-3-methyl-pent-2-ene
I-
H
d.
I
Cl
1-methylcyclopentene
H
H
H
Cl
Cl-
Cl
1-methylcyclopentene
H 2O
e.
2-butene
H 2SO 4
Page 1 of 6
CHEM 109A
CLAS
Alkenes and Reactions of Alkenes - KEY
H
H
O
H 2O
2-butene
+
H
H
O
H
HSO4-
H 2SO 4
H
H
O
H
H
H
O+
O
H
H
H
O+
H
H
H
CH 3OH
f.
H 2SO 4
3-methyl-1-butene
H
O+
H
H
H
3-methyl-1-butene
H
HSO4-
H
O
H
H
H
O+
H
O
O
H
H
H
O+
H
H
CH3CH 2OH
g.
cyclopentene
H 2SO 4
Page 2 of 6
CHEM 109A
CLAS
Alkenes and Reactions of Alkenes - KEY
H
O
O+
CH 3CH2OH
H
H
H 2SO 4
cyclopentene
H
HSO4-
O+
H
O
H
O
O+
H
H
H
H
2. Fill in the missing information in the following reactions
Cl
HCl
a.
3-chlorohexane
2-hexene
or
3-hexene
Br
b.
2-bromo-2-methylbutane
HBr
or
2-methyl-but-2-ene
2-methyl-1-butene
O
c.
propoxycyclohexane
cyclohexene
HO
propanol
H2SO 4
OH
d.
2-pentene
2-pentanol
HO
H 2O
H 2SO 4
also
3-pentanol
Page 3 of 6
CHEM 109A
CLAS
Alkenes and Reactions of Alkenes - KEY
O
e.
1-isopropoxy-2-methylbutane
OH
1-propene
H 2SO 4
Additional Information:
Alkenes (CnH2n, if cyclic CnH2n-2) – hydrocarbons that contain a carbon-carbon
double bond.
Nomenclature
Alkenes (CnH2n): -ene
1. Determine the # of C atoms in the longest continuous chain (parent chain) that
CONTAINS the double bond – Watch for branches!
a. Use a prefix for # of double bonds (EX. diene for 2)
b. A # is not needed to denote position of double bond in ring, b/c ring is
numbered so that double bond is on C #1.
2. Number the chain in the direction that gives the double bond the lowest #.
a. If numbering in both directions gives the same number to the double bond,
number in the direction that gives the lowest number to the alphabetically
first substituent.
3. Substituents are listed in alphabetical order.
O
EX.
2-hexene
2,3-dimethyl-2-pentene
4-hexoxy-1-butene
2-ethylcyclopentene
Structural Details
Vinyl (sp2 Cs in double bond) and allyl (sp3 Cs adjacent to double bond) Cs are in
same plane, double bond is above and below the plane.
Get cis-trans/Z-E (a.k.a. geometric) isomers if vinyl Cs have different groups, and
they are separable. Cis-trans do NOT interconvert b/c to rotate, the π bond would
have to break (∆E C-C π = 62 kcal/mol vs. ∆E C-C σ = 2.9 kcal/mol).
Reactions of Alkenes – See also Summary of Reactions of Alkenes
Page 4 of 6
CHEM 109A
CLAS
Alkenes and Reactions of Alkenes - KEY
Electrophilic Addition Rxns
In general: π e-s of double bond attracted to electrophile (alkene is a Nuc) & get
addition of electrophile to 1 vinyl C and Nuc adds to the other vinyl C.
Can be used to synthesize alkyl halides (EX above), alcohols, ethers, epoxides
and alkanes.
Addition of Hydrogen Halide (HF, HCl, HBr, HI)
Fig 4.1 shows hyperconjugation
Electrophile (proton) adds to least substituted (greatest # of Hs) vinyl C to form the
more stable carbocation and Nuc adds to carbocation → regioselective (one
constitutional isomer is preferred) → a.k.a. Markovnikov addition.
H
H
Br
Br-
hydrobromic acid
2,3-dimethyl-2-butene
3o carbocation intermediate
H
Br
Addition of Water (acid-catalyzed) – a.k.a. hydration
Acid (often H2SO4) required b/c otherwise there is no electrophile. Also need
unreactive solvent to dissolve the alkene (ex. DMSO). Markovnikov add H and –OH.
O
O
H
O
H
water
H
HO
S
O+
OH
H
-O
H
H
O+
H
O
H
H
2,3-dimethyl-2-butene
H
3o carbocation intermediate
O
H
H
H
OH
O
O
sulfuric acid
O+
S
H
H
H
H
H
O
+
HO
H
H
Page 5 of 6
CHEM 109A
CLAS
Alkenes and Reactions of Alkenes - KEY
Addition of Alcohol (acid-catalyzed) – forms an ether
Acid (often H2SO4) required b/c otherwise there is no electrophile. Also need
unreactive solvent to dissolve the alkene (ex. DMSO). Markovnikov add H and –OR.
O
O
H
O
HO
H
S
O+
OH
-O
S
OH
H
O
O
sulfuric acid
H
H
O+
O
H
H
2,3-dimethyl-2-butene
3o carbocation intermediate
H
H
O
H
H
H
O
+
O+
H
O
Page 6 of 6