Alkyl halides substitution

Structurally simple alJkyl halides can sometimes be prepared by reaction of an alkane with Cl2 or Br2 through a radical chain-reaction pathway (Section 5.3). Although inert to most reagents, alkanes react readily with Cl2 or Br2 in the presence of light to give alkyl halide substitution products. The reaction occurs by the radical mechanism shown in Figure 10.1 for chlorination. 

In conversion of alcohols to alkyl halides (substitution), very often, the reaction is accompanied by the formation of some alkenes (elimination). 

Mechanism 6-1 Allylic Bromination 228 Summary Methods for Preparing Alkyl Halides 229 6-7 Reactions of Alkyl Halides Substitution and Elimination 231 6-8 Second-Order Nucleophilic Substitution The Sn2 Reaction 232 Key Mechanism 6-2 The S j2 Reaction 233 6-9 Generality of the SN2 Reaction 234... 

With less hindered alkyl halides hydroxide would not be a good choice as a base for an elimination because it is rather small and still very good at Sn2 substitutions (and even with tertiary alkyl halides, substitution outpaces elimination at low concentrations of hydroxide). So what are good alternatives ... 

Reaction of sodium acetylides with alkyl halides. Substitution vs, elimination... 

Alkyl halide substituted alkane CH3Br bromomethane CH3CH2CI chloroethane alkyl group to which halogen is attached, plus halide CH3Br methyl bromide CH3CH2CI ethyl chloride... 

Table 10.6 Summary of the Reactivity of Alkyl Halides Substitution Reactions in Nucleophilic...

Primary alkyl halides primarily substitution Secondary alkyl halides substitution and elimination Tertiary alkyl halides only elimination... 

Lewis acid AICI3, BF3, HF, etc. RX may be an alkyl halide, substituted alkyl halide, alkene or alcohol. The Lewis acid functions to generate the electrophile that attacks the ring. Since the n-propyl group is activating, the reaction should occur with orientation ortho and para. 

Trialkylstannyl-lithium reacts with secondary alkyl halides (substitution) and with a/S-unsaturated carbonyl compounds (conjugate addition) to give alkyl tin derivatives which may be oxidized with chromic anhydride in pyridine to give a saturated ketone. Applying the procedure to a cycloalkenone, an efficient dialkyl-ative enone transposition can be realized (Scheme 68). ... 

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