Chemistry > General Organic Chemistry > 13.0 Reaction intermediates
General Organic Chemistry
1.0 Introduction
2.0 Classification of organic compounds
3.0 Homologous series
4.0 Nomenclature of hydrocarbons
4.1 The alkanes $(C_nH_{2n+2})$
4.2 The alkenes $(C_nH_{2n})$
4.3 The alkynes $(C_nH_{2n-2})$
4.4 Combined alkenes and alkynes
4.5 Cyclic hydrocarbons
5.0 Nomenclature of compounds containing halogens and nitro groups
6.0 Nomenclature of compounds with functional groups named as suffixes
6.1 Ethers and thioethers
6.2 Alcohols & thiols
6.3 Acids, salts of acids and acid anhydrides
6.4 Esters
6.5 Acid halides
6.6 Amides
6.7 Nitriles
6.8 Aldehydes
6.9 Ketones
6.10 Amines and ammonium salts
7.0 Nomenclature of aromatic compounds
7.1 Halogen and nitro-substituted aromatics
7.2 Carboxylic acids and derivatives
7.3 Phenols and thiophenols
7.4 Aldehydes & Ketones
7.5 Sulfonic acids and sulfonic acid derivatives
7.6 Aromatic amines
7.7 Diazonium ions $\left( {ArN_2^ + } \right)$
8.0 Radicofunctional naming
9.0 Organic reactions
9.1 Substitution or displacement reactions
9.2 Addition reaction
9.3 Elimination reaction
9.4 Rearrangement reactions
10.0 Electrophiles
11.0 Nucleophiles
12.0 Breaking and forming of bonds
13.0 Reaction intermediates
13.1 Carbocations
13.2 Carbanions
13.3 Carbon radical
13.4 Carbenes
13.5 Nitrenes
13.6 Arenium ions
13.7 Benzynes
14.0 Electron displacement effects
15.0 Inductive effects
16.0 Hyperconjugation
17.0 Resonance
18.0 Mesomeric effect
19.0 Electromeric effect
20.0 Inductomeric effect
21.0 Steric inhibition of resonance
22.0 Ortho effect
13.4 Carbenes
4.2 The alkenes $(C_nH_{2n})$
4.3 The alkynes $(C_nH_{2n-2})$
4.4 Combined alkenes and alkynes
4.5 Cyclic hydrocarbons
6.2 Alcohols & thiols
6.3 Acids, salts of acids and acid anhydrides
6.4 Esters
6.5 Acid halides
6.6 Amides
6.7 Nitriles
6.8 Aldehydes
6.9 Ketones
6.10 Amines and ammonium salts
7.2 Carboxylic acids and derivatives
7.3 Phenols and thiophenols
7.4 Aldehydes & Ketones
7.5 Sulfonic acids and sulfonic acid derivatives
7.6 Aromatic amines
7.7 Diazonium ions $\left( {ArN_2^ + } \right)$
9.2 Addition reaction
9.3 Elimination reaction
9.4 Rearrangement reactions
13.2 Carbanions
13.3 Carbon radical
13.4 Carbenes
13.5 Nitrenes
13.6 Arenium ions
13.7 Benzynes
Carbenes are neutral intermediates having bivalent carbon, in which a carbon atom is covalently bonded to two other groups and has two valency electrons distributed between two non-bonding orbitals. When the two electron are spin paired the carbene is a singlet. If the spin of the electrons are parallel, it is a triplet.
(a) Structure: A singlet carbene is thought to possess a bent $sp^2$ hybrid structure in which the paired electrons occupy the vacant $sp^2$ orbital. A triplet carbene can be either bent $sp^2$ hybrid with an electron in each unoccupied orbital or a linear $sp$ hybrid with an electron in each of the unoccupied $p-$ orbital. It has, however, been shown that several carbenes are in a non-linear triplet ground state. However, the di halogen carbenes and carbenes with oxygen, nitrogen and sulfur atoms attached to the bivalent carbon, exist probably as singlets. The singlet and triplet state of a carbene display different chemical behavior. Thus the addition of singlet carbenes to olefinic double bond to form cyclopropane derivatives is much more stereoselective than the addition of triplet carbenes.
(b) Generation: Carbenes are obtained by thermal or photochemical decomposition of diazoalkanes. These can also be obtained by $\alpha$-elimination of a hydrogen halide from a haloform with base, or of a halogen from a gem dihalide with a metal.
(c) Reactions: These add to carbon double bonds and also to aromatic systems and in the later case the initial product rearranges to give ring enlargement products.
When a carbene is generated in a three-membered ring allenes are formed by rearrangement. However, a similar formation at a cyclopropylmethyl carbon gives ring expansion. Carbenes are involved in Reimer-Tiemann reaction.
