496
M. Rashid et al.
7.3.3 Effect of Substituent
Straight chain hydrocarbons: α, β and γ effect: There is remarkable and predictable
shift in chemical-shift value (δ-value) of
13 C-NMR in hydrocarbons which depends
upon position of substituent on carbon-skeleton (α, β, γ, δ etc.). These effects are
called as α, β and γ effects in
13 C-NMR [1–7, 23].
α-effects:
13 C-NMR peak of TMS reference compound appear at zero-ppm chemical
shift (δ-value) of CH 4 is 2.6 (an upfield shift). Replacement of a hydrogen of a
methane with –CH 3 (methyl group) leads to formation of ethane with δ-value 5.7.
Further replacement of one-hydrogen of ethane leads to propane with chemical shift
value (δ) of central carbon 16.0 ppm. Replacement of central carbon-atom of propane
with CH 3 group leads to further downfield-shift about 9.1 ppm and called as α-effect,
i.e. shift in chemical shift value after replacement of its proton by methyl-group (or
substituent) [1–4]
b
a
a
a
a
a
b
b- 25.1 ppm
b- 16.0 ppm
Shift:
16.0
25.1 (ppm)
β-effect: Comparing chemical shift of the methyl atom in ethane, H 3 C-CH 3 (δ,
5.7 ppm) and propane H 3 C–CH 2 –CH 3 (δ, 16 ppm), there is downfield shift of around
~9 ppm after replacement of hydrogen on an adjacent carbon by a methyl group. This
is called as β-effect. Typical value of β-effect is 9.5 ppm [1–4].
H 3 C CH 3
b
a
a
a a
a- 5.7 ppm
b- 16.0 pm
Shift:
5.7
16.0 (ppm)
γ-effect: Effect of substituent bonded to γ-position of carbon atom of molecule under
observations can be calculated in many straight and branched chain hydrocarbons.
Unlike α and β effect, the γ-effect is upfield shift of ~2.5 ppm. Comparison-chart of
γ-effect is given for n-heptane and n-hexane to see γ-effect [1–4].
M. Rashid et al.
7.3.3 Effect of Substituent
Straight chain hydrocarbons: α, β and γ effect: There is remarkable and predictable
shift in chemical-shift value (δ-value) of
13 C-NMR in hydrocarbons which depends
upon position of substituent on carbon-skeleton (α, β, γ, δ etc.). These effects are
called as α, β and γ effects in
13 C-NMR [1–7, 23].
α-effects:
13 C-NMR peak of TMS reference compound appear at zero-ppm chemical
shift (δ-value) of CH 4 is 2.6 (an upfield shift). Replacement of a hydrogen of a
methane with –CH 3 (methyl group) leads to formation of ethane with δ-value 5.7.
Further replacement of one-hydrogen of ethane leads to propane with chemical shift
value (δ) of central carbon 16.0 ppm. Replacement of central carbon-atom of propane
with CH 3 group leads to further downfield-shift about 9.1 ppm and called as α-effect,
i.e. shift in chemical shift value after replacement of its proton by methyl-group (or
substituent) [1–4]
b
a
a
a
a
a
b
b- 25.1 ppm
b- 16.0 ppm
Shift:
16.0
25.1 (ppm)
β-effect: Comparing chemical shift of the methyl atom in ethane, H 3 C-CH 3 (δ,
5.7 ppm) and propane H 3 C–CH 2 –CH 3 (δ, 16 ppm), there is downfield shift of around
~9 ppm after replacement of hydrogen on an adjacent carbon by a methyl group. This
is called as β-effect. Typical value of β-effect is 9.5 ppm [1–4].
H 3 C CH 3
b
a
a
a a
a- 5.7 ppm
b- 16.0 pm
Shift:
5.7
16.0 (ppm)
γ-effect: Effect of substituent bonded to γ-position of carbon atom of molecule under
observations can be calculated in many straight and branched chain hydrocarbons.
Unlike α and β effect, the γ-effect is upfield shift of ~2.5 ppm. Comparison-chart of
γ-effect is given for n-heptane and n-hexane to see γ-effect [1–4].