Exercício de Química Sintética e Organica Mista (248)

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10.26 Bond formation takes place at the end of the diene system to give a bridged bicyclic ring system.
10.27 The two Diels–Alder adducts formed in the reaction of 1,3-pentadiene with acrolein arise by the two
alignments shown:
10.28 Compound B arises by way of a Diels–Alder reaction between compound A and dimethyl
acetylenedicarboxylate. Compound A must therefore have a conjugated diene system.
10.29 The reaction is a nucleophilic substitution in which the nucleophile (C6H5S
�) becomes attached to
the carbon that bore the chloride leaving group. Allylic rearrangement is not observed; therefore,
it is reasonable to conclude that an allylic carbocation is not involved. The mechanism is SN2.
10.30 (a) Solvolysis of in ethanol proceeds by an SN1 mechanism and involves a
carbocation intermediate.
This carbocation has some of the character of a tertiary carbocation. It is more stable and is
therefore formed faster than allyl cation, .
(b) An allylic carbocation is formed from the alcohol in the presence of an acid catalyst.
H2SO4
H2O
CH2 CHCHCH3
OH
3-Buten-2-ol
CH2 CHCHCH3
O
�
H H
H2O�CH2 CHCHCH3
�
CH2 CHCH2
�
(CH3)2C CHCH2Cl
�Cl�
(CH3)2C CH CH2
�
(CH3)2C CH CH2
�
1-Chloro-3-methyl-2-butene
(CH3)2C CHCH2Cl
�CH3CH CHCH2Cl CH3CH CHCH2SSNa
ethanol
1-Chloro-2-butene Sodium
benzenethiolate
2-Butenyl phenyl sulfide
� CH3O2CC CCO2CH3
H2C
CH2
CH2
Compound A
H2C CO2CH3
CO2CH3
Compound B
CH3 CH3
CHO
CH3
CHO
CHO
CH3
CHO
� �and
3-Methylcyclohexene-
4-carboxaldehyde
3-Methylcyclohexene-
5-carboxaldehyde
�
C
CO2CH3
CO2CH3
C
Dimethyl
acetylenedicarboxylate
1,3-Cyclohexadiene Dimethyl bicyclo[2.2.2]octa-
2,5-diene-2,3-dicarboxylate
CO2CH3
CO2CH3
CONJUGATION IN ALKADIENES AND ALLYLIC SYSTEMS 243