[budullewraagh]

this week's exam the last question asked for a synthesis of meta chloro propylbenzene.

in notes, halides were considered to be weak as electron donating and electron withdrawing groups, but basically mostly neutral between the two.

my proposed synthesis was the chlorination of benzene with AlCl3 as a catalyst, then meta addition of propane by using propyl chloride and AlCl3.

the answer key says to add propyl chloride with AlCl3 and then to add chlorine, reduce the C=O with NaBH4 (i would have just done a wolff-kishner) then eliminate with H2SO4 and hydrogenate with H2 on a Pd catalyst.  other routes would involve using TsCl on the -OH and eliminating with a strong base, then hydrogenating with H2 on a Pd catalyst.

i recognize how the "correct" answer would yield more chlorine in the meta position, but even if each of those 5 steps have a 75% yield, the overall yield is only 23.7%.  i believe that competition between ortho/para and meta addition of propyl chloride would result in at least 25% meta product, especially considering the fact that Cl is the second most electronegative halogen.

does anyone agree?

[GCT]

I think the problem with your proposal in substituting the alkane through fridel crafts is the reaction will progress further as the alkane groups are very much activating.  Additional substitutions will take place, so you'll have trouble stopping the reaction at just one reaction

[movies]

GCT's concern is a good one.  Sometimes you can control the second addition, but not always.  The bigges problem I see is that halobenzenes are actually pretty good ortho/para directors even though they deactivate the ring.  For example, nitration of chlorobenzene gives a 30:1:69 product distribution of ortho:meta:para.  The only reliable way to get meta direction is with a resonance withdrawing group or a blocking group.

Also, alkylation with propyl chloride would likely lead to a fair amount of rearragend iso-propyl alkylation products since you would form a primary carbocation from propyl chloride.  This is another reason why propionyl chloride is prefered (F-C acylation instead of alkylation).

There are probably faster ways of getting to the product from the F-C acylation product than what is on the answer key though.  In fact, hydrogenation with H₂/Pd/C in the presence of aryl halides is a risky proposition.  Wolff-Kishner conditions might be too harsh as well.

[budullewraagh]

i brought up the problem that H2/Pd posed to my professor, but he didn't seem to understand the problem.  can you substantiate this statement?

[movies]

In some cases H₂ and Pd/C can be used to reduce an aryl halide, e.g. bromobenzene to benzene.

See:
Pandey, P. N.; Purkayashta, M. L. Synthesis, 1982, 876-878.

I don't actually have all that much experience with Pd/C hydrogenation reactions, so maybe this isn't as big a problem as I think it is.  Perhaps the conditions in the above paper are more specialized than I though.  It's definitely something to consider though.