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Theory and problems of organic chemistry - Meislich H.

Meislich H., Nechamkin H.,Sharefkin J., Hademenos G. Theory and problems of organic chemistry - Mc Graw-Hill, 1999. - 478 p.
ISBN 0-07-134165-X
Download (direct link): theoryandproblms1999.djv
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/ 2
CH=0 0=CH VCH=CH 1
H2C CH2 I o, l- CH,
I I | 7 u -
H2c /CH2 2. H20(Zn) 2 , /2
= 0=CH =
1,5-Cyclooctadiene
6.5 SUBSTITUTION REACTIONS AT THE ALLYLIC POSITION
Allylic carbons are the ones bonded to the doubly bonded Cs; the Hs attached to them are called allylic Hs.
ci2 + h2c=chch3 high-emperalur^ H2C=CHCH2C1 + HC1 Br2 + H2C=CHCH, !-cen--r-^ H2C=CHCH,Br + HBr
of Br2
The low concentration of Br2 comes from /V-bromosuccinimide (NBS).
,0
NBr + HBr
NH + Br2

NBS product of Succinimide
bromination
S02Cl2 + H2C=CHCH, peroLe* H2C=CHC'H2C] + HCI + so2
Sulfuryl
chloride
These halogenations are like free-radical substitutions of alkanes (see Section 4.4). The order of reactivity of H-abstraction is
allyl > 3 > 2 > 1 > vinyl
Problem 6.35 Use the concepts of (a) resonance and (b) extended n orbital overlap (delocalization) to account for the extraordinary stability of the ally 1-typc radical.
therefore the ally 1-type radical has considerable resonance energy (Section 2.7) and is relatively stable.
(b) The three Cs in the allyl unit arc ,vp2-hybridized and each has a p orbital lying in a common plane (Fig. 6-7). These three p orbitals overlap forming an extended system, thereby delocalizing the odd electron. Such delocalization stabilizes the allyl-type free radical.
Problem 6.36 Designate the type of each set of Hs in CH3CH=CHCH2CH2CH(CH3)2 (e.g. 3, allylic, etc.) and show their relative reactivity toward a Br- atom, using (1) for the most reactive, (2) for the next, etc. A
Labeling the Hs as
C=Cї
(a) Two equivalent resonance structures can be written:
c=cc -> c=c
(</) (M ) (<) (</) (e) (,/)
CH1CH=CHCH2CH2CH(CH1)2
we have: (a) 1\ allylic (2); (6) vinylic (6); (c ) 2'. allylic (1); (d) 2 (4); (e) 3 (3); ( 1 (5).
Fig. 6-7
6.6 SUMMARY OF ALKENE CHEMISTRY
PREPARATION
1. Dehydrohalogenation of RX
RCHXCH,, RCH,CH,X + ale. KOH)
2. Dehydration of ROH
RCHOHCH,, rch,ch,oh + h2so41
I
heat
3. Dehalogenation of vie - Dihalide
RCHXCH2X + Zn
4. Dehydrogenation of Alkanes
RCH,CH3, heat, Pt/Pd
5. Reduction of RG=CR
RC=CH + H2 or Na/C2H5OH
RCH=CH
PROPERTIES 1. Addition Reactions
(a) Hydrogenation
Heterogeneous: H2/Pt
Rh(PPti3],CI
Homogeneous: H2------------RCH2CH3
Chemical: (BH3), ^^
(b) Polar Mechanism
+ X,
RCHXCH2X
(X = CI, Br)
+ x2, h2o
+ h2so4-
RCH(OH)CH2X
RCH(OH)CH3
RCH(0S03H)CH3
+ BH3, H202 + NaOH-
RCH2CH2OH
+ dil. cold KMn04RCH(OH)CH2OH + hot KMnCX. - RCOOH + CO,
+ RCO,H
-il*
RCHCH,
V
+ RCOjH, H,0 + + HF, HCMe,
-RCH(OH)CH2OH RCH2CH2CMe3 + 03, Zn, H20-* rch=o + CH2=0
(c) Free-Radical Mechanism + HBr-* RCH,CH,Br + CHCI3 RCH2CH2CC1,
+ H+ or BF3> polymer
2. Allylic Substitution Reactions
RCH;CH=CH2 + X2?-*
RCHXCH=CH,
Supplementary Problems
Problem 6.37 Write structures for the following:
(a) 2,3-dimethyl-2-pentene
(b) 4-chloro-2,4-dimethyl-2-pentene
(c) allyl bromide
(d) 2,3-dimethylcyclohexene
(e) 3-isopropyl-1-hexene
(/) 3-isopropyl-2,6-dimethyl-3-heptene
CH3
.! /=(
(a) H3C0=CCH2CH, (d) ( )CH,
CH,
CH, CH,
I I
(b) H,cc=chcCH, (<) CH2=CHCHCH2 CH2CH3
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