Halogenation of higher alkanes

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Halogenation of higher alkanes
According to statistics presuming equal
probability of reacting
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But experimentally ..
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• Thus, there are two players
• Statistical factor
• Stability of the radical (3gt2gt1)

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Selectivity of bromine
(99)
(traces)
F (99) (1)
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Transition state more similar to
reagents
products radical
character of CH3 group less

more Any factor that stabilizes radicals
stabilizes the transition state and increases the
reaction rate (favoring that particular product)
In conclusion Br2 although less reactive is the
best reagent
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Shape of the Me radical
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Isomerism
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Stereoisomers
Constitutional isomers
Stereoisomers
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Chirality
- Chiral molecules are not superposable to their
mirror image - The mirror image of the chiral
molecule is called enantiomer
Examples of chiral objects screwdriver, golf
club, shoe, ear, car
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Lets consider 2-butanol
Lets focus on this C atom
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A criterion for deciding about chirality
The presence of a C atoms bearing 4 different
groups
H
stereocenter

CH3-C-CH2CH3
OH
Interconversion of one enantiomer into the other
cannot be done (unless cleaving and reforming
bonds)
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If two groups are identical, the molecule is
achiral (superposable to its mirror image)
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Chirality test (mirror plane)
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Nomenclature (R and S Cahn, Ingold, Prelog)
R rectus (right) S sinister (left)
Lets consider 2-butanol
To assign the R,S label we need to sequence
all the groups attached to the chiral C atom
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Rule 1 Priority is assigned on the basis of
the atomic . (H has the lowest)
a
d
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Rule 2 For two identical atoms (Me and Et both
have C) priority is assigned at the first point
of difference
c
a
d
b
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Rule 3 Lets consider the C-d bond as a
 handle  and by holding it, rotate the
molecule towards your face. See if the sequence
a,b,c defines a clockwise R or
counterclockwise S rotation
(R)-2-butanol
(S)-2-butanol
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Rule 4
Groups containing double or triple bonds are
counted as both atoms were duplicated or
triplicated
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Exercise Assign R or S designation to each of
the following compounds
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Exercise Assign R or S designation to each of
the following compounds
S R
S