91Ó°ÊÓ

A significant amount of energy is needed to stretch a bond than to bend it. The absorption bands of stretching vibrations are observed in the functional group region $(\mathbf{4000}\mathbf{-}\mathbf{1400}){\mathbf{cm}}^{-1}$and those of the bending vibrations are observed in the fingerprint region$(\mathbf{1400}\mathbf{-}\mathbf{600}){\mathbf{cm}}^{-1}$ .

a. An absorption band at $1250{\mathrm{cm}}^{−1}$due to C-O stretching vibration is present for the ester which is the first compound and absent for the ketone which is the second compound.

b. An absorption band at $720{\mathrm{cm}}^{−1}$due to in-phase rocking of the five adjacent methylene groups would be present for the second compound, heptane and absent for the first compound, methylcyclohexane.

c. An absorption band at 3650 to role="math" localid="1652422159839" $3200{\mathrm{cm}}^{−1}$is present due to the O-H stretching is present for the first molecule which is an alcohol and second molecule which is an ether.

d. An absorption band at 3500 to role="math" localid="1652422155668" $3300{\mathrm{cm}}^{−1}$due to an N-H stretching vibration is present for the first compound which is an amide and absent for the second compound which is an ester.

e. The secondary alcohol which is the second compound has an absorption band at 1385 to role="math" localid="1652422209400" $1365{\mathrm{cm}}^{−1}$for the methyl group. The primary alcohol which is the first compound does not have a methyl group and an absorption band.

f. The trans isomer has a C-H bending absorption at 980 to $960{\mathrm{cm}}^{−1}$. The cis isomer has an absorption band at 730 to $675{\mathrm{cm}}^{−1}$. In addition, a weak C=C absorption band at 1680 to $1600{\mathrm{cm}}^{−1}$is present for cis isomer and absent for the trans isomer, as the trans isomer is symmetrical.

g. The C=O absorption band possess a larger wave number ,$1740{\mathrm{cm}}^{−1}$for the ester which is the first compound. The ketone which is the second compound possess a wave number of$1720{\mathrm{cm}}^{−1}$.

h. The C=O absorption band comprise a larger wavenumber for $\mathrm{β},\mathrm{γ}$-unsaturated ketone and the value is $1720{\mathrm{cm}}^{−1}$. The $\mathrm{Î\pm },\mathrm{β}$unsaturated ketone has a value of $1680{\mathrm{cm}}^{−1}$, as the double bonds of the latter are conjugated.

i. The alkene has an absorption band at 1680 to $1600{\mathrm{cm}}^{−1}$due to the C=C stretching vibration and at 3100 to $3020{\mathrm{cm}}^{−1}$due to an ${\mathrm{sp}}^2$C-H stretching vibration that is not present in the alkyne. The alkyne has an absorption band at 2260 to $2100{\mathrm{cm}}^{−1}$that is not present in the alkene.

j. An absorption band at 2820 and $2720{\mathrm{cm}}^{−1}$due to aldehyde C-H stretching vibration is present for the aldehyde which is the first compound, and the second compound which is a ketone would not exhibit this peak.

k. An absorption band at $1600{\mathrm{cm}}^{−1}$and $1500{\mathrm{cm}}^{−1}$(aromatic ring stretching vibrations) and at 3100 to $3020{\mathrm{cm}}^{−1}$$\left({\text{sp}}^{\text{2}}\text{C-H}\right)$stretching vibration is present for the compound with benzene ring and absent for the compound with cyclohexane ring. An absorption band at 2960 to $2850{\mathrm{cm}}^{−1}$due to ${\mathrm{sp}}^3$C-H stretching vibration is present for the compound with cyclohexane ring and absent for the compound with benzene ring.

l. Absorption band at $990{\mathrm{cm}}^{−1}$and $910{\mathrm{cm}}^{−1}$due to an${\mathrm{sp}}^2$ C-H bending vibration that is present for terminal alkene and absent for internal alkene.