Question

Please help answer questions. Thank you!

1. A) Why is the C=0 stretch in the IR so much stronger than the C=C stretch of a simple alkene hydrocarbon (2 points) B) Why is the CNC stretch of 1-methoxy-1-propeneso much stronger than the CNC stretch of 1- butene. Justify your answer with the use of a resonance structure. (2 points). C) Explain why the C=0 stretch of 1-butanal requires more energy (higher frequency) than the CEO stretch of acrolein. Justify your answer with a resonance structure (2 points). D) Explain why methane (natural gas) is considered to be a green house gas (4 points).

Answer 1

1) a) the stretching frequency depends on two things. The mass of atoms connected by a bond and the force constant of the bond. Mass has almost neglible effect but the force constant is based on the electronegativity difference of atoms connected by bond. Thus C and O are having more electrongetivity difference that C and C ( which is equal to 0) . That is why C=O bonds are having higher stretching frequency.

b) when electron donating group like OMe group donates electron on one carbon of C=C, naturally the electronegativity difference will rise and that will lead to stronger bond and thus frequency will be high

C) when C=O bond is in conjugation with alkene bond, the double bond character of C=O is lost and single bond character becomes significant. Thus bond order decreases and so the bond strength. Thus butanal will have stronger bond stretching than C=O streych of acrolein with is having conjugation.

D) methane can actually trap heat by absorbing heat and distributing among its degree of freedom like vibration, rotational and translational motion. Thus ehatever substance trap heat , they are twrmed as green house particles.

MO g) orme (electron donating group) 9 an* o - aerolein Double bond cao Single bond C-o.