1. The mechanism just described is an example of an SN1 process.
Mechanistic notation
The mechanism just described is an example of an SN1 process.
SN1 stands for substitution-nucleophilic- unimolecular.
The molecularity of the rate-determining step defines the molecularity of the overall reaction. 9

2. Mechanistic notation
The molecularity of the rate-determining step defines the molecularity of the overall reaction.
(CH3)3C + O H
Rate-determining step is unimolecular dissociation of alkyloxonium ion. 9

3. Effect of Alcohol Structure on Reaction Rate6

4. slow step is: ROH2+  R+ + H2O
The more stable the carbocation, the faster it is formed.
Tertiary carbocations are more stable than secondary, which are more stable than primary, which are more stable than methyl.
Tertiary alcohols react faster than secondary, which react faster than primary, which react faster than methanol. 22

5. Hammond's Postulate
If two succeeding states (such as a transition state and an unstable intermediate) are similar in energy, they are similar in structure.
Hammond's postulate permits us to infer the structure of something we can't study (transition state) from something we can study (reactive intermediate). 23

6. carbocation formation carbocation capture
proton transfer
ROH2 +
ROH RX 21

7. carbocation formation
Rate is governed by energy of this transition state.
Infer structure of this transition state from structure of state of closest energy; in this case the nearest state is the carbocation.
carbocation capture R+
proton transfer
ROH2 +
ROH RX 21

8. 4. 13 Reaction of Primary Alcohols with Hydrogen Halides
4.13 Reaction of Primary Alcohols with Hydrogen Halides. The SN2 Mechanism 6

9. Preparation of Alkyl Halides
25°C
(CH3)3COH + HCl
(CH3)3CCl + H2O
78-88%
OH + HBr
80-100°C Br
+ H2O
73%
120°C
CH3(CH2)5CH2OH + HBr
CH3(CH2)5CH2Br + H2O
87-90% 4

10. Preparation of Alkyl Halides
Primary carbocations are too high in energy to allow SN1 mechanism. Yet, primary alcohols are converted to alkyl halides.
Primary alcohols react by a mechanism called SN2 (substitution-nucleophilic-bimolecular).
120°C
CH3(CH2)5CH2OH + HBr
CH3(CH2)5CH2Br + H2O
87-90% 4

11. Two-step mechanism for conversion of alcohols to alkyl halides:
The SN2 Mechanism
Two-step mechanism for conversion of alcohols to alkyl halides:
(1) proton transfer to alcohol to form alkyloxonium ion
(2) bimolecular displacement of water from alkyloxonium ion by halide 27

12. Example
120°C
CH3(CH2)5CH2OH + HBr
CH3(CH2)5CH2Br + H2O 4

13. Step 1: Proton transfer from HBr to 1-heptanol
Mechanism
Step 1: Proton transfer from HBr to 1-heptanol O
CH3(CH2)5CH2 .. .. : + H Br : .. H
fast, bimolecular H Br : .. – +
CH3(CH2)5CH2 O : + H
Heptyloxonium ion 9

14. Step 2: Reaction of alkyloxonium ion with bromide ion.
Mechanism
Step 2: Reaction of alkyloxonium ion with bromide
ion. H O : +
CH3(CH2)5CH2 Br : .. – +
slow, bimolecular O H : Br : ..
CH3(CH2)5CH2 +
1-Bromoheptane 9

15. CH2
OH2 + Br –
CH3(CH2)4 CH2
proton transfer
ROH
ROH2 + RX 31

16. 4.14 Other Methods for Converting Alcohols to Alkyl Halides6

17. Phosphorus tribromide PBr3 + 3ROH  3RBr + H3PO3
Reagents for ROH to RX
Thionyl chloride
SOCl2 + ROH  RCl + HCl + SO2
Phosphorus tribromide
PBr ROH  3RBr H3PO3 33

18. (pyridine often used instead of K2CO3)
Examples
SOCl2
CH3CH(CH2)5CH3
CH3CH(CH2)5CH3
K2CO3 Cl OH
(81%)
(pyridine often used instead of K2CO3)
PBr3
(CH3)2CHCH2OH
(CH3)2CHCH2Br
(55-60%) 34