Due to the very electron rich nature of phenol it is possible to nitrate phenol using gentle conditions. One method is to use a solution of nitric acid (6 % w/w) with a solution of the phenol in ethylene dichloride with some tributyl ammonium bromide. Now I do not like the sound of this system, I would suggest instead using aliquat 336 in aromatic kerosene with dilute nitric acid. My dislike of the method presented in Organic Process Research & Development 2003, 7, 95−97 by Ashutosh V. Joshi, Mubeen Baidoosi, Sudip Mukhopadhyay, and Yoel Sasson is that the solvent they choose to use is carcinogenic. I would advocate using a solution of the phenol in aromatic kerosene (How about Solvesso 150ND) do the reaction with the nitric acid and then extract the nitrated phenols with sodium hydroxide before making the sodium hydroxide solution acidic to allow the nitrophenols to be recovered.
You should be able to understand the mechanism of the nitration of the phenol using what you know already about the reactions of the 2-tert-butylanisole and toluene. Now we have to move onto something else.
What happens if we have an arene bearing a group which withdraws electron density by the inductive pull effect but donates by the resonance effect ? This might sound like a rather strange group but we already have some which do this.
We have anisole which has a slight electron withdrawing effect by means of its inductive pull and a strong electron donation effect by means of resonance. But a more clear case is chlorobenzene. The chlorine atom has a strong inductive pull and it donates by resonance. Here is a diagram showing the resonance and inductive effects.
Now many years ago there was a wonder chemical, a non toxic insecticide which is about as toxic to humans (acute effect) as aspirin. It was regarded as a safe alternative to things like lead, arsenic, thallium and nicotine for use on food crops. It offered humans a means of expunging insects from farmland and the extermination of disease carrying insects such as body lice and malaria mosquitos. Sounds great but we later found that there were some horrible problems.
- Many of the insect pests became immune to its effects
- The agent killed off lots of useful insects such as honey bees
- The agent breaks down into a long lived substance which is toxic to some mammals (such as bats)
- The agent breaks down to something which is a xenoestrogen, the agent itself might also be active as a xenoestrogen.
In the USA there have been problems at Lake Apopka (Florida) as a result of xenoestrogens entering the lake, these have harmed birds and alligators (Cynthia V. Rider et. al. Environ Toxicol Chem. 2010 September ; 29(9): 2064–2071. doi:10.1002/etc.233).
The compound in question is DDT, DDT was made by the electrophilic aromatic substitution reaction of chlorobenzene with choral using an acid catalyst. In the first stage a molecule of chlorobenzene reacts with choral (trichloroacetaldehyde) to form the first of the new C-C bonds.
Then a second molecule of chlorobenzene will react with the 2,2,2-trichloro-1-(4-chlorophenyl)ethan-1-ol. This will protonate to form a resonance stabilized cation.
We finish off the DDT synthesis with a second reaction of a chlorobenzene molecule on the new cationic intermediate.
Now we have formed a molecule of DDT, here is the crystal structure of DDT in case you are interested to know what it looks like.
It is important to keep in mind that the synthesis of DDT is not able to make a perfectly pure product, commercial DDT used to contain some of the ortho,para and the ortho,ortho isomers. Here is the molecular structure of the ortho, para isomer of DDT.
Now we have been going through the synthesis of a molecule which has been banned. It might never have been banned if some sectors of society had used DDT in a more responsible manner. If we ignore the harmful effects on wildlife (the man reaoson for the ban) and concentrate on the problem of DDT resistance in insects then it is likely that if DDT had never been used for agriculture but had been only used for controlling insects which transmit disease to humans (eg Malaria control) then DDT might still be in use. I reason that the widespread outdoor use of DDT created a legion of DDT resistant insects while causing the various problems.
In case you think that the chemistry required for the formation of DDT from chloral and chlorobenzene has gone away, it has not. The synthesis of bisphenol A from acetone and phenol is using close to identical chemistry. Bisphenol A is made by the reaction of phenol and acetone using an acid catalyst
When you are ready for the final part of the lesson on aromatic substitutions please click here.