Exercício de Química Sintética e Organica Mista (116)

Prévia do material em texto

(e) For the 1,1-dimethylpropyl cation to be formed by a process involving a hydride shift, the
starting alcohol must have the same carbon skeleton as the 1,1-dimethylpropyl cation.
Although the same carbon skeleton is necessary, it alone is not sufficient; the alcohol must
also have its hydroxyl group on the carbon atom adjacent to the carbon that bears the migrat-
ing hydrogen. Thus, 3-methyl-1-butanol cannot form a tertiary carbocation by a single hy-
dride shift. It requires two sequential hydride shifts.
( f ) 2,2-Dimethyl-1-propanol can yield a tertiary carbocation by a process involving a methyl
shift.
5.36 (a) Heating an alcohol in the presence of an acid catalyst (KHSO4) leads to dehydration with
formation of an alkene. In this alcohol, elimination can occur in only one direction to give a
mixture of cis and trans alkenes.
Br
CHCH2CH3 CHCH3
Br
CH
OH
KHSO4
heat
Cis–trans mixture
CH3
CH3
H�
CH3C
CH3
��
CH3C
2,2-Dimethyl-1-propanol
CH2 CH2CH3O
H
H
CH3
CH3
CH3C CH2OH
CH3
H
H�
CH3CHCH
CH3
CH3CHCH2CH2OH
CH3
H
CH3C
� ��
3-Methyl-1-butanol
CH2 CHCH3O
H
H
CH3
CH3CCH2CH3
CH3
C
H
H�
�H2O
CH3
CH3 OH
CHCH3
H
CH3C
CH3
C
��
CH3
3-Methyl-2-butanol
CHCH3 CH2CH3
�
CH3
HOCH2CCH2CH3
H
CH3
CCH2CH3
H
H�
HO
H
CH2
CH3
CCH2CH3
�
CH3
2-Methyl-1-butanol
H
CH3C CHCH3
CH3
CH3CCH2CH3
�
CH3 CH3 OH
HOCH2CCH2CH3has same carbon skeleton as
H
and
STRUCTURE AND PREPARATION OF ALKENES: ELIMINATION REACTIONS 111