Question

Nucleophilic Substitution Reactions Lab (S_N1 and S_N2 reaction mechanisms)

Below are my lab results for S_N1 and S_N2 reactions. This first column of the chart gives the ten starting material halides used in this experiment. The second column gives the reaction time for S_N1 mechanisms with silver nitratate in ethanol solution. The third column gives the reaction time for S_N2 mechanisms with sodium iodide.

In theory, primary alkyl halides are expected to react faster in S_N2 mechanisms, and tertiary alkyl halides are expected to react faster in S_N1 mechanisms. However, my experimental results showed that many of the primary alkyl halides tested actually reacted faster during S_N1 mechanisms. I believe the reason for this has something to do with resonance stabilization of the subsequent carbocations, but I do not completely understand what all of that means.

So other than experimental errors, what might be the cause of my experimental results contradicting the theoretical expectations?

Thank you for your help!!

2-bromobutane (2˚)	0:20	5:56
Bromobenzene (1˚)	6:56	N/A
1-bromobutane (1˚)	0:48	0:24
Cyclohexyl bromide (1˚)	0:06	11:00
1-bromo-2-methylpropane (1˚)	0:42	6:31
1-chlorobutane (1˚)	7:02	6:02
Crotyl chloride (1˚)	0:03	0:35
Benzyl chloride (1˚)	0:27	0:31
2-chlorobutane (2˚)	6:22	8:46
2-chloro-2-methylpropane (3˚)	0:01	N/A

Answer 1

From given table all the primary alcohols donot react faster in SN1 reactions than in SN2 reactions. Some are faster in SN1 and Some are faster in SN2.

1-chlorobutane and 1-bromobutane have faster rate in SN2 than in SN1.

1-bromo-2-methylpropane (1˚), Benzyl chloride (1˚) they faster rate in SN1 than in SN2. Because in these two reactions the produced primary carbocation rearranged to more stable carbocation by alkyl shift or hidride ion shift or by resonance.   

First step in SN1 reactions to]p to produce carbocation. CH Slow +Br CH CH benzyl carbocation has total four resonance structures hence it is more stable. CH Slow Hidride Hjc .+ Br ion shift less stable primary carbocation more stable tertiary carbo cation