tertiary alkyl halide and reaction rate

[Unlearner]

can someone help clarify why tertiary alkyl halide (tert-butyl chloride) react with HCl and alcohol much quicker than a primary alkyl halide (methyl halide)?

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[G1SG2]

can someone help clarify why tertiary alkyl halide (tert-butyl chloride) react with HCl and alcohol much quicker than a primary alkyl halide (methyl halide)?

The slowest and rate determining step in the above reaction (which is an SN1 reaction-you can't have an SN2 with a tertiary alkyl halide, and the alcohol sucks as a nucleophile to undergo SN2 with the primary alkyl halide) is the formation of the carbocation (CH3)3C-Cl-->(CH3)3C+ (after the leaving group, in this case, Cl, leaves).

At this point, you have a tertiary carbocation: (CH3)3C+Compare this to the methyl carbocation CH3+. Now, alkyl groups are electron donating. The tertiary carbocation has THREE alkyl groups donating electrons to it via hyperconjugation (the three methyl groups attached to the central C). This stabilizes the positive charge on the carbocation, stabilizing the transtion state and thus making the reaction proceed relatively faster. On the other hand, the methyl carbocation is TERRIBLE-it has no electron donating groups to help it out. It stinks. Tertiary carbocations are better than secondary carbocations, which are better than primary carbocations, which are better than the methyl carbocation (the last two really stink).

I'm not too sure about the purpose of the HCl, though.

 

[loveoforganic]

The primary/methyl actually go through an SN2 mechanism following protonation of the alcohol.

 

[G1SG2]

The primary/methyl actually go through an SN2 mechanism following protonation of the alcohol.

? They're alkyl halides...unless you mean after the alcohols are synthesized, and THEN they are protonated. Is that what you meant?

 

[loveoforganic]

I misread completely, disregard!

 

[Unlearner]

"The slowest and rate determining step in the above reaction (which is an SN1 reaction-you can't have an SN2 with a tertiary alkyl halide, and the alcohol sucks as a nucleophile to undergo SN2 with the primary alkyl halide) is the formation of the carbocation (CH3)3C-Cl-->(CH3)3C+ (after the leaving group, in this case, Cl, leaves). "


but the question is asking for the fastest reaction...so how come the slowest and rate determining step is the fastest reaction? i'm a bit confused.

 

[G1SG2]

The slowest and rate determining step in the above reaction (which is an SN1 reaction-you can't have an SN2 with a tertiary alkyl halide, and the alcohol sucks as a nucleophile to undergo SN2 with the primary alkyl halide) is the formation of the carbocation (CH3)3C-Cl-->(CH3)3C+ (after the leaving group, in this case, Cl, leaves).


but the question is asking for the fastest reaction...so how come the slowest and rate determining step is the fastest reaction? i'm a bit confused.

No no, that is the slowest step amongst all the steps for the TERTIARY alkyl halide. SN1 reactions happen in two steps. SN2 reactions happen in one step. For the SN1 reaction, the formation of the carbocation is the slowest step out of the two steps JUST FOR THE SN1 REACTION. It doesn't mean that the slowest step in the tertiary SN1 reaction is slower than the step for the methylchloride reaction.

Suppose you have two groups, groups A and B. Group A has Joe, Danny, and Mike. Group B has only Peter.

Now, suppose I tell you that in Group A, Joe is fatter than both Danny and Mike. This means that Joe is the fattest kid in Group A. Does that mean he is fatter than Peter in Group B? No! We don't know anything about Peter. All we know is that Joe is the fattest kid in Group A, but not necessarily fatter than Peter, who is in Group B. Same goes for the reactions. The formation of the carbocation is the slowest step for THAT SN1 REACTION-it doesn't have to do with the SN2 reaction of the methyl chloride.

Maybe I shouldn't have used the word "slowest step" (even though it is) for the formation of the carbocation. Think of it as the toughest step. We need to get it going. The three alkyl/methyl groups on the teritary carbon is t-butyl stabilize the carbocation by donating electrons via hyperconjugation (remember, alkyl groups are electron donating). They make this tough step easier for us, stabilizing the transition state, and anything that stabilizes the transition state (remember, transition states are highly reactive, unstable species) makes the reaction go faster. Don't worry about the formation of the carbocation being slow for this question-just think of it as being a tough step, and our friendly electron donating alkyl/methyl groups on t-butyl make this easier on us.

Sorry if my example is ******ed lol, but does that help claify things?

 

[Dr Gerrard]

So why is the two step Sn1 faster than the Sn2 which all happens in one step?

We know the primary would not go an Sn1 so it has to do an Sn2.
However, the tertiary would not go an Sn2 and has to go an Sn1.

 

[Unlearner]

No no, that is the slowest step amongst all the steps for the TERTIARY alkyl halide. SN1 reactions happen in two steps. SN2 reactions happen in one step. For the SN1 reaction, the formation of the carbocation is the slowest step out of the two steps JUST FOR THE SN1 REACTION. It doesn't mean that the slowest step in the tertiary SN1 reaction is slower than the step for the methylchloride reaction.

Suppose you have two groups, groups A and B. Group A has Joe, Danny, and Mike. Group B has only Peter.

Now, suppose I tell you that in Group A, Joe is fatter than both Danny and Mike. This means that Joe is the fattest kid in Group A. Does that mean he is fatter than Peter in Group B? No! We don't know anything about Peter. All we know is that Joe is the fattest kid in Group A, but not necessarily fatter than Peter, who is in Group B. Same goes for the reactions. The formation of the carbocation is the slowest step for THAT SN1 REACTION-it doesn't have to do with the SN2 reaction of the methyl chloride.

Maybe I shouldn't have used the word "slowest step" (even though it is) for the formation of the carbocation. Think of it as the toughest step. We need to get it going. The three alkyl/methyl groups on the teritary carbon is t-butyl stabilize the carbocation by donating electrons via hyperconjugation (remember, alkyl groups are electron donating). They make this tough step easier for us, stabilizing the transition state, and anything that stabilizes the transition state (remember, transition states are highly reactive, unstable species) makes the reaction go faster. Don't worry about the formation of the carbocation being slow for this question-just think of it as being a tough step, and our friendly electron donating alkyl/methyl groups on t-butyl make this easier on us.

Sorry if my example is ******ed lol, but does that help claify things?

I was thinking that the reaction (alkyl halide and HCl & -OH) was SN2 because there was a strong base (alcohol) so I assumed that the primary alkyl halide would be favored thus react faster. so does this mean that -OH is not a strong base in this reaction? to be more specific, what is the mechanism for this reaction (i forgot)?

 

[G1SG2]

I was thinking that the reaction (alkyl halide and HCl & -OH) was SN2 because there was a strong base (alcohol) so I assumed that the primary alkyl halide would be favored thus react faster. so does this mean that -OH is not a strong base in this reaction? to be more specific, what is the mechanism for this reaction (i forgot)?

OH- is NOT alcohol! That's hydorixde ion. Alcohol is R-OH. Okay, so you've got OH- in the mixture, right? That totally changes things. OH- is a strong base. R-OH (alcohol) is a weak base. So, we have OH-, a strong base and a good nucleophile which CAN proceed via an SN2 reaction (ONLY WITH THE METHYL CHLORIDE-remember you can NEVER have an SN2 reaction with a tertiary alkyl halide due to severe steric hindrance). Let's look at both situations again, this time considering HCl and OH-.

First of all, if you have HCl (a strong acid) and OH- (a strong base) TOGETHER in the beaker, you're going to have an acid base reaction. The acid will protonate the OH-, giving you water H2O. That screws over your good nucleophile. Now you have water, a poor nucleophile. You need to understand that you won't get an SN2 reaction with a poor nucleophile (you need a GOOD nucleophile for an SN2 reaction for a backside attack). This leaves us with SN1. Okay, so now we know we're only going to have an SN1 reaction with both the tertiary chloride and the methyl chloride.

Now, you have a tertiary alkyl halide and water, a poor nucleophile. What did we say about tertiary alkyl halides? You can NEVER have an SN2 reaction with a tertiary alkyl halide because it's too bulky/too hindered for a backside attack that you get with an SN2. Cross off SN2 immediately ANYTIME you see a tertiary alkyl halide. We're STILL going to have our SN1 reaction. The Cl is going to leave, giving us our carbocation (CH3)3C-Cl-->(CH3)3C+. The poor carbon has a positive charge because the Cl left it all alone. BUT remember-we have our three friendly alkyl groups that donate electorns to the C+ via hyperconjugation (alkyl groups are electron donating), so things aren't that bad.

Let's look at the methyl chloride, CH3-Cl. Now, we WOULD'VE had an SN2 reaction with the OH-, because hydroxide is a strong base. HOWEVER this is a trick question-you also said you have HCl in the beaker. OH-=strong base. HCl=strong acid (anytime you see HCl, H2SO4, HNO3, ANY of the 7 strong acids in a reaction in organic chemistry-immediately write H+ on top). So, OH- has a choice between attack the backside of the carbon in our methyl chloride in an SN2 reaction, or it can go for the H+ in HCl. Do you think it's going to want the delta plus on the backside of carbon, or the FULL plus on H+? Of course it's going to go for the full plus! 20 dollars is better than 10 dollars 😎

So, we already said we're going to have an acid base reaction with the HCl (strong acid) and OH- (strong base), and like we said, uh oh, we screwed over our OH-, the good nucleophile-we converted it to water. What is water? A POOR nucleophile. You cannot have an SN2 reaction with poor nucleophiles. You COULD'VE had it with JUST OH-, like you said, but the trick is to realize that you also have bad boy HCl lying around, and being the strong acid that it is it's going to protonate your OH- and ruin your plans for an SN2. Both will proceed via an SN1 reaction, so the only thing that makes the tertiary alkyl halide proceed faster is the fact that it's + charge is stabiliezd by the alkyl groups. Tertiary carbocations are MUCH better than methyl carbocations (these poor guys don't have any generous electron donating groups to stabilize that positive charge on the carbon).

SO, the important thing to realize is that you would've had an SN2 reaction with the methyl chloride and OH- like you yourself said, BUT you have to understand that there's HCl lying around, whose going to turn your good nucleophile, OH- into a poor nucleophile, H2O by protonating it, thus ruining your chances of an SN2. All you're left with now is an SN1 reaction. You're job is much easier now that you've narrowed it down to which reaction. All you have to do now is compare the two SN1 style. The main thing to remember about SN1 reactions is that you form a carbocation in the first step. Anything that can donate electrons and help out that carbocation speeds up the reaction. Tertiary carbocations have three friends helping it (3 electron donating alkyl groups) while the methyl cation stinks and has no alkyl friends to help it out. Does this clear up things?

 

[sv3]

so did the OP mistake the OH for ROH in their original post?

jeez this one is giving me a headache

also, Pookiez88, is it pretty much a rule that alcohols are too weak to be the nucleophiles for Sn2 rxns? I know I've seen reactions where they are the nucleophiles in Sn1s, but can't think off the top of my head where they have been the nucleophile's in Sn2s.......

thanks

 

[G1SG2]

so did the OP mistake the OH for ROH in their original post?

jeez this one is giving me a headache

also, Pookiez88, is it pretty much a rule that alcohols are too weak to be the nucleophiles for Sn2 rxns? I know I've seen reactions where they are the nucleophiles in Sn1s, but can't think off the top of my head where they have been the nucleophile's in Sn2s.......

thanks

Yeah, I think the OP just confused OH- for ROH. Alcohols are poor nucleophiles; they generally don't partake in SN2 reactions (but can do SN1 reactions).

 

[sv3]

well knowing its OH makes it alot easier

i still am not quite sure what the heck i would have done with the ROH (the Cl- from HCl is prolly a worse nucleophile than an alchol so i would have predicted a Sn2 and Sn1 for the methyl and tertiary compounds respectively...which would be wrong now that I know ROH ain't up to the Sn2 thing)

Good to know about ROHs and Sn1

cheers

 

[minutemen11]

Yeah, I think the OP just confused OH- for ROH. Alcohols are poor nucleophiles; they generally don't partake in SN2 reactions (but can do SN1 reactions).

Ummm I think I may be ******ed or something but.. R-OH + HBR is an SN2 mechanism.

I do agree with you that the acid-base rxn will form H2O. I know that the reactions will proceed SN1 for the first one because it is tertiary, but the second one is a methy halide (very strongly favored for sn2, it is the worst possible substrate you could use for sn1). This issue im having is, H2O is a protic solvent (which would strongly favor sn1), however it can also dissociate to hydronium and hydroxide ions (meaning an sn2 rxn is possible). What effect takes higher precedence; substrate effects or solvent effects? Substrate effects would make the methly halide proceed Sn2 (though very very slowly whenever H2O dissociates into hydroxide) while solvent effects would make the methyl halide proceed via SN1 (again very very slowly because while solvation occurs, it is nearly impossible for the cloride to leave and result in a primary carbocation therby giving a SN1).

There are no hard set rules, you need to look at all the players involved. Frankly, the answer to this question is probably best determined experimentally and would never appear as an mcat question; only with the tertiary Carbocation can you be 100% sure it will proceed Sn1 (due to steric hindrance AND solvation), the methly chloride part of the question would not appear.

Let me know if i'm being a total idiot...

 

[G1SG2]

Ummm I think I may be ******ed or something but.. R-OH + HBR is an SN2 mechanism.

I do agree with you that the acid-base rxn will form H2O. I know that the reactions will proceed SN1 for the first one because it is tertiary, but the second one is a methy halide (very strongly favored for sn2, it is the worst possible substrate you could use for sn1). This issue im having is, H2O is a protic solvent (which would strongly favor sn1), however it can also dissociate to hydronium and hydroxide ions (meaning an sn2 rxn is possible). What effect takes higher precedence; substrate effects or solvent effects? Substrate effects would make the methly halide proceed Sn2 (though very very slowly whenever H2O dissociates into hydroxide) while solvent effects would make the methyl halide proceed via SN1 (again very very slowly because while solvation occurs, it is nearly impossible for the cloride to leave and result in a primary carbocation therby giving a SN1).

There are no hard set rules, you need to look at all the players involved. Frankly, the answer to this question is probably best determined experimentally and would never appear as an mcat question; only with the tertiary Carbocation can you be 100% sure it will proceed Sn1 (due to steric hindrance AND solvation), the methly chloride part of the question would not appear.

Let me know if i'm being a total idiot...

Yeah, it could've been an SN2 mechanism because we have an unhindered backside and all but we do have a polar protic solvent AND the Ka for the autoionization for water is very small. So we have lots of H2O and little OH-.