Diazo compounds
Diazo compounds are a group of organic-chemical compounds with the general structural formula R 1 R 2 C = N = N; R 1 and R 2 are carbon-containing radicals or hydrogen. The simplest diazo compound is diazomethane . The diazo compounds were discovered by Peter Grieß in 1858 .
properties
Diazo compounds are isolable, but very reactive, toxic compounds that often serve as synthesis intermediates . Simple diazo compounds are volatile and explosive. Diazo compounds have a very good leaving group with N 2 (formation of gaseous nitrogen ). Reactions with diazo compounds must be carried out at low temperatures, since diazo compounds decompose very easily (elimination of N 2 ). Aromatic diazo compounds are usually more stable than aliphatic ones . They show a stable, internal dipole character, which can be described by two mesomeric boundary structures; one of them is a ylid .
Use in reactions
Diazo compounds can be used as alkylating agents e.g. B. are used for esterification or for the formation of ethers , due to their high price and because they are difficult to handle, but mostly only in the laboratory.
So z. B. Diazomethane as a methylating agent for compounds with enough acidic hydrogen:
- RCO 2 H + H 2 CN 2 → RCO 2 CH 3 + N 2
In the first step of the reaction, the acidic proton is transferred to the diazomethane, which forms a methanediazonium ion [H 3 C − N 2 ] + . A nucleophile or, as in this case, an anion RCO 2 - reacts further with elimination of N 2 .
Alcohols with substituents that show an −I effect, which increase the acidity of the OH group, can react relatively easily with diazomethane. However, non-activated alcohols cannot be methylated directly or only in the presence of silica gel.
Diazo compounds are also important as a source of carbenes , which are formed by splitting off N 2 .
- RR'CN 2 → RR'C: + N 2
Manufacturing
Due to their instability, diazo compounds are mostly used in situ , i.e. H. directly before the actual reaction.
- Diazomethane from N -methyl- N -nitroso-p-toluenesulfonamide
- Tol-SO 2 −N (NO) CH 3 + KOH → H 2 CN 2 + Tol-SO 3 - K + + H 2 O
The diazomethane formed is continuously distilled off.
- Nitrosation of glycine ester (see azo coupling )
- ROOC-CH 2 -NH 2 + NO + → ROOC-CHN 2
- Oxidation of hydrazones
- RR′C = N − NH 2 + OxMean → RR′CN 2
This variant is suitable for aromatic compounds. As an oxidizing agent, for. B. PbO 2 , Ag 2 O, CrO 2 or HgO are used.
literature
- R. Brückner: reaction mechanisms. 3rd edition, Spektrum Akad. Verlag, 2004, ISBN 3-8274-1579-9 .
- Peter Grieß: Preliminary note on the action of nitrous acid on amidinitro and aminitrophenyl acid . In: Annals of Chemistry and Pharmacy . tape 106 , no. 1 , 1858, p. 123-125 , doi : 10.1002 / jlac.18581060114 .
- Walter Bayer: textbook of the org. Chemistry. 24th edition. 2004.