Carbon–carbon bond

A carbon–carbon bond is a covalent bond between two carbon atoms.[1] The most common form is the single bond: a bond composed of two electrons, one from each of the two atoms. The carbon–carbon single bond is a sigma bond and is formed between one hybridized orbital from each of the carbon atoms. In ethane, the orbitals are sp3-hybridized orbitals, but single bonds formed between carbon atoms with other hybridizations do occur (e.g. sp2 to sp2). In fact, the carbon atoms in the single bond need not be of the same hybridization. Carbon atoms can also form double bonds in compounds called alkenes or triple bonds in compounds called alkynes. A double bond is formed with an sp2-hybridized orbital and a p-orbital that is not involved in the hybridization. A triple bond is formed with an sp-hybridized orbital and two p-orbitals from each atom. The use of the p-orbitals forms a pi bond.

Chains and branching

Carbon is one of the few elements that can form long chains of its own atoms, a property called catenation. This coupled with the strength of the carbon–carbon bond gives rise to an enormous number of molecular forms, many of which are important structural elements of life, so carbon compounds have their own field of study: organic chemistry.

SubstitutionAtCarbon
2,2,3-trimethylpentane

Branching is also common in C−C skeletons. Different carbon atoms can be described by the number of carbon neighbors they each have:

In "structurally complex organic molecules", it is the three-dimensional orientation of the carbon–carbon bonds at quaternary loci which dictates the shape of the molecule.[2] Further, quaternary loci are found in many biologically active small molecules, such as cortisone and morphine.[2]

Synthesis

Carbon–carbon bond-forming reactions are organic reactions in which a new carbon–carbon bond is formed. They are important in the production of many man-made chemicals such as pharmaceuticals and plastics.

Some examples of reactions which form carbon–carbon bonds are aldol reactions, Diels–Alder reaction, the addition of a Grignard reagent to a carbonyl group, a Heck reaction, a Michael reaction and a Wittig reaction.

The directed synthesis of desired three-dimensional structures for tertiary carbons was largely solved during the late 20th century, but the same ability to direct quaternary carbon synthesis did not start to emerge until the first decade of the 21st century.[2]

Bond strengths and lengths

Relative to most bonds, a carbon–carbon bond is very strong.[3]

C–C bond Molecule Bond dissociation energy (kcal/mol)
CH3−CH3 ethane 90
C6H5−CH3 toluene 102
C6H5−C6H5 biphenyl 114
CH3C(O)−CH3 acetone 84
CH3−CN acetonitrile 136
CH3−CH2OH ethanol 88

The values given above represent bond dissociation energies that are commonly encountered; occasionally, outliers may deviate drastically from this range. In the highly congested hexakis(3,5-di-tert-butyl)ethane, the bond dissociation energy to form the stabilized triarylmethyl radical is only 8 kcal/mol.[4] On the opposite extreme, the central carbon–carbon single bond of diacetylene is very strong at 160 kcal/mol, as the single bond joins two carbons of sp hybridization.[5] Carbon–carbon multiple bonds are generally stronger; the double bond of ethylene and triple bond of acetylene have been determined to have bond dissociation energies of 174 and 230 kcal/mol, respectively.[6]

A typical carbon–carbon single bond has a length of 154 pm, while a typical double bond and triple bond are 134 pm and 120 pm, respectively. Also a consequence of its severe steric congestion, hexakis(3,5-di-tert-butyl)ethane has a greatly elongated central bond with a length of 167 pm. On the other hand, a very short triple bond of 115 pm has been observed for the iodonium species [HC≡C–I+Ph][CF3SO3], due to the strongly electron-withdrawing iodonium moiety.[7]

See also

  • An extensive list is presented here: list of carbon–carbon bond-forming reactions
  • The chemistry of carbon bonded to other elements in the periodic table:

References

  1. ^ Dembicki, Harry (2016-10-06). Practical Petroleum Geochemistry for Exploration and Production. Elsevier. p. 7. ISBN 9780128033517.
  2. ^ a b c Quasdorf, Kyle W.; Overman, Larry E. (2014). "Review: Catalytic enantioselective synthesis of quaternary carbon stereocentres". Nature (paper). 516 (7530): 181–191. Bibcode:2014Natur.516..181Q. doi:10.1038/nature14007. PMC 4697831. PMID 25503231.closed access publication – behind paywall
  3. ^ Yu-Ran Luo and Jin-Pei Cheng "Bond Dissociation Energies" in CRC Handbook of Chemistry and Physics, 96th Edition.
  4. ^ Rösel, Sören; Balestrieri, Ciro; Schreiner, Peter R. (2017). "Sizing the role of London dispersion in the dissociation of all-meta tert-butyl hexaphenylethane". Chemical Science. 8 (1): 405–410. doi:10.1039/c6sc02727j. ISSN 2041-6520. PMC 5365070. PMID 28451185.
  5. ^ "NIST Webbook".
  6. ^ Blanksby, Stephen J.; Ellison, G. Barney (April 2003). "Bond Dissociation Energies of Organic Molecules". Accounts of Chemical Research. 36 (4): 255–263. CiteSeerX 10.1.1.616.3043. doi:10.1021/ar020230d. ISSN 0001-4842. PMID 12693923.
  7. ^ 1927-, Streitwieser, Andrew, (1992). Introduction to organic chemistry. Heathcock, Clayton H., 1936-, Kosower, Edward M. (4th ed.). Upper Saddle River, N.J.: Prentice Hall. p. 574. ISBN 978-0139738500. OCLC 52836313.
Acetoacetic ester synthesis

Acetoacetic ester synthesis is a chemical reaction where ethyl acetoacetate is alkylated at the α-carbon to both carbonyl groups and then converted into a ketone, or more specifically an α-substituted acetone. This is very similar to malonic ester synthesis.

Auwers synthesis

The Auwers synthesis is a series of organic reactions forming a flavonol from a coumarone. This reaction was first reported by Karl von Auwers in 1908.

The first step in this procedure is an acid catalyzed aldol condensation between benzaldehyde and a 3-cyclooxapentanone to an o-hydroxychalcone. Bromination of the alkene group gives a dibromo-adduct which rearranges to the flavonol by reaction with potassium hydroxide.

Cadiot–Chodkiewicz coupling

The Cadiot–Chodkiewicz coupling in organic chemistry is a coupling reaction between a terminal alkyne and a haloalkyne catalyzed by a copper(I) salt such as copper(I) bromide and an amine base. The reaction product is a 1,3-diyne or di-alkyne.

The reaction mechanism involves deprotonation by base of the terminal alkyne proton followed by formation of a copper(I) acetylide. A cycle of oxidative addition and reductive elimination on the copper centre then creates a new carbon-carbon bond.

Claisen condensation

The Claisen condensation is a carbon–carbon bond forming reaction that occurs between two esters or one ester and another carbonyl compound in the presence of a strong base, resulting in a β-keto ester or a β-diketone. It is named after Rainer Ludwig Claisen, who first published his work on the reaction in 1887.

Coupling reaction

A coupling reaction in organic chemistry is a general term for a variety of reactions where two fragments are joined together with the aid of a metal catalyst. In one important reaction type, a main group organometallic compound of the type R-M (R = organic fragment, M = main group center) reacts with an organic halide of the type R'-X with formation of a new carbon-carbon bond in the product R-R' Richard F. Heck, Ei-ichi Negishi, and Akira Suzuki were awarded the 2010 Nobel Prize in Chemistry for developing palladium-catalyzed cross coupling reactions.Broadly speaking, two types of coupling reactions are recognized:

Heterocouplings couple two different partners, such as in the Heck reaction of an alkene (RC=CH) and an alkyl halide (R'-X) to give a substituted alkene (RC=CR').

Homocouplings couple two identical partners, as in the Glaser coupling of two acetylides (RC≡CH) to form a dialkyne (RC≡C-C≡CR).

Eclipsed conformation

In chemistry an eclipsed conformation is a conformation in which two substituents X and Y on adjacent atoms A, B are in closest proximity, implying that the torsion angle X–A–B–Y is 0°. Such a conformation exists in any open chain, single chemical bond connecting two sp3-hybridised atoms, and it is normally a conformational energy maximum. This maximum is often explained by steric hindrance, but its origins sometimes actually lie in hyperconjugation (as when the eclipsing interaction is of two hydrogen atoms).

In the example of ethane in Newman projection it shows that rotation around the carbon-carbon bond is not entirely free but that an energy barrier exists. The ethane molecule in the eclipsed conformation is said to suffer from torsional strain and by a rotation around the carbon carbon bond to the staggered conformation around 12.5 kJ/mol of torsional energy is released.

Glaser coupling

The Glaser coupling is a type of coupling reaction. It is by far the oldest acetylenic coupling and is based on cuprous salts like copper(I) chloride or copper(I) bromide and an additional oxidant like oxygen. The base in its original scope is ammonia. The solvent is water or an alcohol.

The reaction was first reported by Carl Andreas Glaser in 1869.

Heck reaction

The Heck reaction (also called the Mizoroki-Heck reaction) is the chemical reaction of an unsaturated halide (or triflate) with an alkene in the presence of a base and a palladium catalyst (or palladium nanomaterial-based catalyst) to form a substituted alkene. It is named after Tsutomu Mizoroki and Richard F. Heck. Heck was awarded the 2010 Nobel Prize in Chemistry, which he shared with Ei-ichi Negishi and Akira Suzuki, for the discovery and development of this reaction. This reaction was the first example of a carbon-carbon bond-forming reaction that followed a Pd(0)/Pd(II) catalytic cycle, the same catalytic cycle that is seen in other Pd(0)-catalyzed cross-coupling reactions. The Heck reaction as a way to substitute alkenes.

Knoevenagel condensation

The Knoevenagel condensation (pronounced [ˈknøːvənaːɡl̩]) reaction is an organic reaction named after Emil Knoevenagel. It is a modification of the aldol condensation.A Knoevenagel condensation is a nucleophilic addition of an active hydrogen compound to a carbonyl group followed by a dehydration reaction in which a molecule of water is eliminated (hence condensation). The product is often an α,β-unsaturated ketone (a conjugated enone).

In this reaction the carbonyl group is an aldehyde or a ketone. The catalyst is usually a weakly basic amine. The active hydrogen component has the form

Z–CH2-Z or Z–CHR–Z for instance diethyl malonate, Meldrum's acid, ethyl acetoacetate or malonic acid, or cyanoacetic acid.

Z–CHR1R2 for instance nitromethane.where Z is an electron withdrawing functional group. Z must be powerful enough to facilitate deprotonation to the enolate ion even with a mild base. Using a strong base in this reaction would induce self-condensation of the aldehyde or ketone.

The Hantzsch pyridine synthesis, the Gewald reaction and the Feist–Benary furan synthesis all contain a Knoevenagel reaction step. The reaction also led to the discovery of CS gas.

Kowalski ester homologation

The Kowalski ester homologation is a chemical reaction for the homologation of esters.

This reaction was designed as a safer alternative to the Arndt–Eistert synthesis, avoiding the need for diazomethane. The Kowalski reaction is named after its inventor, Conrad J. Kowalski.

Liebeskind–Srogl coupling

The Liebeskind–Srogl coupling reaction is an organic reaction forming a new carbon–carbon bond from a thioester and a boronic acid using a metal catalyst. This reaction was invented by and named after Jiri Srogl from the Academy of Sciences, Czech Republic, and Lanny S. Liebeskind from Emory University, Atlanta, Georgia, USA. The metal catalyst depicted below uses TFP = tris(2-furyl)phosphine as an additional ligand and CuTC = copper(I) thiophene-2-carboxylate as a co-metal catalyst. The overall reaction scheme is shown below.

Malonic ester synthesis

The malonic ester synthesis is a chemical reaction where diethyl malonate or another ester of malonic acid is alkylated at the carbon alpha (directly adjacent) to both carbonyl groups, and then converted to a substituted acetic acid. The major drawback of malonic ester synthesis is that the alkylation stage can also produce dialkylated structures. This makes separation of products difficult and yields lower.

Michael reaction

The Michael reaction or Michael addition is the nucleophilic addition of a carbanion or another nucleophile to an α,β-unsaturated carbonyl compound. It belongs to the larger class of conjugate additions. This is one of the most useful methods for the mild formation of C–C bonds. Many asymmetric variants exist.

In this scheme the R and R' substituents on the nucleophile (a Michael donor) are electron-withdrawing groups such as acyl and cyano making the methylene hydrogen acidic forming the carbanion on reaction with base B:. The substituent on the activated alkene, also called a Michael acceptor, is usually a ketone making it an enone, but it can also be a nitro group.

Palladium-catalyzed coupling reactions

Palladium-catalyzed coupling reactions comprise a family of cross-coupling reactions that employ palladium complexes as catalysts. It is an active area of research and applications in homogeneous catalysis. In 2010, the Nobel Prize in Chemistry was awarded to Richard F. Heck, Ei-ichi Negishi and Akira Suzuki for their work on palladium-catalyzed cross couplings in organic synthesis.

Reformatsky reaction

The Reformatsky reaction (sometimes spelled Reformatskii reaction) is an organic reaction which condenses aldehydes or ketones, with α-halo esters, using a metallic zinc to form β-hydroxy-esters:

The organozinc reagent, also called a 'Reformatsky enolate', is prepared by treating an alpha-halo ester with zinc dust. Reformatsky enolates are less reactive than lithium enolates or Grignard reagents and hence nucleophilic addition to the ester group does not occur. The reaction was discovered by Sergey Nikolaevich Reformatsky.

Some reviews have been published.

Thorpe reaction

The Thorpe reaction is a chemical reaction described as a self-condensation of aliphatic nitriles catalyzed by base to form enamines. The reaction was discovered by Jocelyn Field Thorpe.

Ullmann reaction

The Ullmann reaction or Ullmann coupling is a coupling reaction between aryl halides and copper. The reaction is named after Fritz Ullmann.

Ullmann-type reaction is the copper-promoted conversion of aryl halides to aryl ethers, aryl thioethers, and aryl amines. Ullmann-type reactions are comparable to Buchwald-Hartwig reactions.

Wurtz reaction

The Wurtz reaction, named after Charles Adolphe Wurtz, is a coupling reaction in organic chemistry, organometallic chemistry and recently inorganic main group polymers, whereby two alkyl halides are reacted with sodium metal in dry ether solution to form a higher alkane:

2 R–X + 2 Na → R–R + 2 Na+X−Other metals have also been used to effect the Wurtz coupling, among them silver, zinc, iron, activated copper, indium and a mixture of manganese and copper chloride. The related reaction dealing with aryl halides is called the Wurtz-Fittig reaction.This can be explained by the formation of free radical intermediate and its subsequent disproportionation to give alkene. The Wurtz reaction occurs through a free radical mechanism that makes possible side reactions producing alkene products.

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