1) Different hybridizations give rise to different vibrational frequencies. Terminal alkynes have C-H stretching frequencies of...

1) Different hybridizations give rise to different vibrational frequencies. Terminal
alkynes have C-H stretching frequencies of about 3300 cm-1; alkene C-H stretching
frequencies are around 3100 cm-1, and alkane C-H stretching frequencies are
around 2900 cm-1. Explain these observations

Expert Solution

1) sp hybridized carbons have 50% s-character, sp² hybridized carbons have 33% s-character and sp³ hybridized carbons have 25% s-character. More the s-character a bond has, more tightly the bond would be held by it.

Hence, sp C-H is the strongest.The next is sp² C-H which is stronger than sp³ C-H bond.

Greater the bond strength, higher is the vibrational frequecy for the bond. Hence, alkyne C-H has higher stretching frequency (3300 cm^-1) than alkene C-H (3100 cm^-1?); which in turn, has higher stretching frequency than alkane C-H (2900 cm^-1?).

2) Esters have higher stretching frequencies than ketone due to the presence of electronegative oxygen atom which has negative inductive effect and hence, stiffens C=O.

In esters though resonance is possible which should decrease vibrational frequency(because conjugation decreases vibrational frequency) but it doesn't happen so. Here, -I effect dominates +R effect, which decreases the electron density in the C=O and stiffens the bond. As a result, stretching frequency increases.

queen_honey_blossom answered 3 months ago

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