In this lab you will explore the effect structure has on the rate of SN2/ SN1 re

Question

In this lab you will explore the effect structure has on the rate of SN2/ SN1 reactions. The substances to be testedare listed below

1)1-chlorobutane   (2)   1-bromobutane     
(3) 2-chlorobutane   (4)2-bromobutane(5) 2-chloro- 2-methylpropane         (6) 2-bromo- 2-methylpropane(7) 1-bromo- 2-methylpropane (8) bromocyclohexane(9) bromobenzene    (10) ß-Bromostyrene   (11) 2-iodobutane

.
General Procedure: In each test, add 5 drops of haloalkane to a test-tube, and then add 1 mL of solution (1M NaI/acetone for the SN2reactions reactions). Mix by swirling vigorously. Record the time it takes to see a precipitate.

3. What was the effect of the halide being directly attached to an aryl/vinyl carbon on the SN2reactivity? Which compounds are they? Did your observations agree with what you learned in class?
(b) What was the effect of the halide being directly attached to an aryl/vinyl carbon on the SN1reactivity? Did your observations agree with what you learned in class?

4. Both 1-bromobutane and 1-bromo- 2-methylpropane are primary bromides. Can you explain thedifference in their SN2 reactivity, if there was any?

5. What would be the specific mathematical effect on the reaction rate if you carried out thesodium iodide-in-acetone reactions on the alkyl halides using an iodide solution half asconcentrated? (“Slower” or “faster” is not specific enough.)

Explanation / Answer

Bromobenzene (arylhailde) and beta-bromostyrene (vinyl hailde) both are non-reactive under both SN1 and SN2 conditions.Reasons are as follows:

(a)
For an SN2 reaction to occur, the nucleophile must attack from behind the carbon which is hindered by the bulky ring in arylhailde.

There are geometric problems too. To perform backside attack on a vinyl halide, attack from within the plane of the molecule, causes steric issues with the other carbon of the double bond. This isn’t aproblem when the carbon is just a CH2 group, but here is larger group attached then the reaction is very sterically hindered.

(b) An SN1 reaction cannot occur since the all the carbons in the benzene ring have sp2 hybridized orbitals and going sp2 to sp costs a lot more energy than going sp3 to sp2 .    Aryl and vinyl carbocations are heavily disfavored both because of the high cost of going sp2 to sp and because there’s no resonance stabilization like there would be in allylic/benzylic carbocations.

The other reason for this unreactivity is that the -orbital of the double bond overlaps with the p-orbitals of the halogen atom to form delocalized cloud of the -electrons with the result that C-X bond attains a partial double bond character. The partial double bond character of the C-X bond results in strengthening of the bond and hence it is difficult to break as compared to single bond. Both of these effects inhibit nucleophilic substitution reactions of either the SN1 or SN2 type, thus net reactivity of the molecule is considerably less than that of saturated alkyl halides.

Related Questions

Navigate

• Browse (All)
• Browse I
• Subjects

Integrity-first tutoring: explanations and feedback only — we do not complete graded work. Learn more.