A nucleophilic attack proceeds the fastest with SN1 or SN2?

[UIUCstudent]

I've had this question twice now, once in an EK chapter question and again in a BR stand-alone question. The question is somewhere along the line of "A nucleophilic attack proceeds most rapidly with" and you have a list of choice of good LG (Br or Cl) and either a tertiary or primary carbon.

EK says this is one of those wtf question the MCAT can throw at you, just narrow your choices to the better leaving group, Br, and move on. It then claims SN1 proceeds faster than SN2.

BR on the other hand says SN2 is faster.

Not sure what to go with here..but I'm take my gamble with BR

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[tttgo]

I would assume sn2 is generally faster because it's concerted, whereas sn1 isn't.
http://bnaddy.columbiastate.edu/Organic Chemistry 1/Sn1 vs Sn2.pdf
However, maybe in some cases, like on a tertiary carbon with hindrance, sn2 is slower. Your question wasn't exactly clear on that (you said tertiary or primary carbon).

 

[Rabolisk]

What were the exact questions? Your question is too general to answer.

 

[kickerkb]

Sn1 reaction is two steps:rate is dependent only on the concentration of the substrate
1.) Slow step- Formation of the Carbocation; the leaving group breaks away on its own leaving the carbocation.
2.) Nucleophile attacks the carbocation

Sn2 Reaction has one step and rate is dependent on both the substrate and the nucleophile:
1.) Nucleophile attacks the substrate at the leaving group, knocking it off while bonding to the substrate.

Via these descriptions I would say Sn2 using primary and secondary carbons would move faster.

As far as particular leaving groups, Nucleophilicity decreases going up and to the right on the periodic table.