reasoning. (3 pts) .) There are 3 different alkenes, A, B, and C, with the molec

Question

reasoning. (3 pts) .) There are 3 different alkenes, A, B, and C, with the molecular formula CsHo. Each one reacts with hydrogen to ield n-pentane (CsH2). Alkene A has IR absorption bands at 1642, 990, and 911 cm'; alkene B has IR absorption bands at 964 cml, and no absorption in the 1600-1700 cm region; and alkene C has absorptions bands at 1658 and 695 cml. Identify the three alkenes. (6 pts) Alkene A structure: Alkene B structure Alkene C structure 4.) The frequency of the O-H stretching absorption of an alcohol in solution changes as the alcohol solution is diluted. Briefly explain. (3 pts)

Explanation / Answer

A is 1- pentene. A shows the C=C absorption at 1642 characteristic of C=C absorption of monosubstituted alkene.

The vinyl out of plane bendings are characterised by 990,911 absorptions.

B is trans 2- pentene. the trans double bond is identified by the weak/ no absorption in the region 1600. the trans oop(out of plane bend is identified at 965 .

C is cis-2-pentene , shows characteristic di substituted C=C str at 1658 and the cis oop (out of plane ) bending at 695 .

Q2) When the spectrum is taken neat or in concentrated solution, we observe the broad H- bonded O-H stretching from 3000 to 3600 centred at 3300. This usually overlaps the C-H str. region However as the solvent is added, on dilution the H- bond beomes less, and the free O-H stretching can be observed as sharp peaks, separate from the C-H str.

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