Chlorination of Arenes to give Chloroarenes
Description: Treatment of an aromatic (such as benzene) with chlorine (Cl2) and a Lewis acid such as AlCl3 or FeCl3 leads to formation of the chlorinated aromatic.
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What is the order of this reaction ? Does it follow second order kinetics or third order kinetics? Please explain.
Thank you!
(For the non catalyzed reaction) – “The chlorination of phenolic ethers and anilides and the chlorination of a number of aromatic hydrocarbons exhibit clean second-order kinetics”. https://pubs.acs.org/doi/abs/10.1021/ja01576a025
Measuring reaction rates with a Lewis acid catalyst is more complex since there tends to be a biphasic mixture. See https://pubs.acs.org/doi/10.1021/ja01060a017
What is the driving factor that makes Cl2 want to give electrons to the catalyst?
It is a Lewis base, albeit a weak one, and the catalyst is a strong Lewis acid. AlCl3 for example has less than a full octet, so donation of a lone pair from Cl to the Al is quite facile. This has the effect of weakening the Cl-Cl bond.
In the reaction of 3-cyclohexenyl prop-1-ol…..with bromine (Br2, electrophille).. ……….i NOW correctly understand in the first step of mechanism the nucleophille is the pi-bond, NOT one of the the lone pairs on the hydroxyl group. A professor explained the pi bond trumps an oxygen lone pair in nucleophicity. The story is a 3 membered bromonium bridge cation is formed and then an oxygen lone pair orbital comes around to attack (forming an oxonium ion). This effects a cyclohexane ring closing. My question is: Why doesnt (or maybee it -is- a minor product) the Br- anion….(remember him… created from the ‘inital’ attack?) stereo anti-attack one of the carbon bridge heads of the bromonium ion (+) created? Like in typical Br2 anti-add stereo of an alkene? “They’re” always saying how the large radii halogens are great nucleophiles because their chrages are polarized/spread out over the atom (e.g. Iodine being best nucleophile). Why is the oxygen lone pair orbital the better nucleophile here?
Or do i -still- have the mech wrong? Damn. A halohydrin being formed here right?
Andrew- seeking understanding, aspiring orgo chemist
Aren’t the examples reactions para-attacks, not ortho?
Yes they are – there’s a note next to the example saying (+ ortho) meaning that they also give the ortho product, but it’s not shown.