Bond length Totally Explained

Everything about Bond Length totally explained

In molecular geometry, bond length or bond distance is the average distance between nuclei of two bonded atoms in a molecule.

Explanation

Bond length is inversely related to bond order, when more electrons participate in bond formation the bond will get shorter. Bond length is also inversely related to bond strength and the bond dissociation energy, as a stronger bond is also a shorter bond. In a bond between two identical atoms half the bond distance is equal to the covalent radius. Bond lengths are measured in molecules by means of x-ray diffraction. A set of two atoms sharing a bond is unique going from one molecule to the next. For example the carbon to hydrogen bond in methane is different from that in methyl chloride. It is however possible to make generalizations when the general structure is the same.

Bond lengths of carbon with other elements

A table with experimental single bonds for carbon to other elements is given below. Bond lengths are given in picometers. By approximation the bond distance between two different atoms is the sum of the individuales covalent radii (these are given in the chemical element articles for each element). As a general trend, bond distances decrease across the row in the periodic table and increase down a group. This trend is identical to that of the atomic radius.

H	106 - 112	group 1
Be	193	group 2
Mg	207	group 2
B	156	group 13
Al	224	group 13
In	216	group 13
C	154 - 120	group 14
Si	186	group 14
Sn	214	group 14
Pb	229	group 14
N	147 - 210	group 15
P	187	group 15
As	198	group 15
Sb	220	group 15
Bi	230	group 15
O	143 - 215	group 16
S	181 - 255	group 16
Cr	192	group 6
Se	198 - 271	group 16
Te	205	group 16
Mo	208	group 6
W	206	group 6
F	134	group 17
Cl	176	group 17
Br	193	group 17
I	213	group 17
Table 1. Bond distance* of carbon to other elements in picometers*

Bond lengths in organic compounds

The actual bond length between two atoms in a molecule depends on such factors as the orbital hybridisation and the electronic and steric nature of the substituents. The carbon carbon bond length in diamond is 154 pm which is also the largest bond length that exists for ordinary carbon covalent bonds.
   Unusually long bond lengths do exist. In one, tricyclobutabenzene, a bond length of 160 pm is reported. The current record holder is another cyclobutabenzene with length 174 pm based on X-ray crystallography . In this type of compounds the cyclobutane ring would force 90° angles on the carbon atoms connected to the benzene ring where they ordinarily have angles of 120°.
   The existence of a very long C-C bond length of up to 290 pm is claimed in a dimer of two tetracyanoethylenedianions although this concerns a 2-electron-4-center bond . This type of bonding has also been observed in dimers of neutral phenalene dimers. The bond lengths of these so-called pancake bonds are up to 305 pm.
   Shorter than average carbon carbon bonds distances are also possible, alkenes and alkynes have bond lengths of respectively 133 and 120 pm due to increased s-character of the sigma bond.In benzene all bonds have the same length: 139 pm. In carbon carbon single bonds increased s-character is also notable in the central bond of diacetylene (137 pm) and that of a certain tetrahedrane dimer (144 pm).
   In propionitrile the cyano group withdraws electrons also resulting in a reduced bond length (144 pm). Squeezing a CC bond is also possible by application of strain. An unusual organic compound exists called In-Methylcyclophane with a very short bond distance of 147 pm for the methyl group being squeezed between a trypticene and a phenyl group. In an in silico experiment a bond distance of 136 pm is estimated for neopentane locked up in fullerene . The smallest theoretical CC single bond obtained in this study is 132 pm for a hypothetical adamantane like molecule. In the same study it's estimated that for ethane it takes 2.8 kJ/mol to stretch the CC bond by 5 pm from its equilibrium value and only 3.5 kJ/mol to squeeze it by the same amount. on the other hand, stretching and squeezing by 15 pm requires 21.9 and 37.7 kJ/mol.

Bond lengths in organic compounds
C-H	length (pm)	C-C	length (pm)	Multiple bonds	length (pm)
sp³-H	110	sp³-sp³	154	Benzene	140
sp²-H	109	sp³-sp²	150	Alkene	134
sp-H	108	sp²-sp²	135	Alkyne	120
		sp³-sp	146	Allene	130
		sp²-sp	143
		sp-sp	120

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