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Bond length is 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, however, there are also few exceptions (Ex, H-H and H-O, the latter one has longer and stronger 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 individual 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.
Bond lengths in organic compounds
The actual bond length between two atoms in a molecule depends on such factors as the orbital hybridization 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 90Â° angles on the carbon atoms connected to the benzene ring where they ordinarily have angles of 120Â°.
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 131 pm for a hypothetical tetrahedrane derivative.
In the same study, it is 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.
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Answers:C is wrong. The resonance structures which one can draw do have alternating single and double bonds, but the reality is somewhere in between. The ring is actually formed by six identical bonds, each somewhere in between a single bond and a double bond in character.
Answers:3 is wrong benzene normally undergoes substitution reactions. it has no double bonds so it does not undergo the typical reaction of that bond
Answers:Honestly, it's not really best qualified as either alkene or alkane. While benzene does have unsaturation (double bonds), the behavior of those olefins is extremely different from normal alkenes. That's what aromaticity is all about -- benzene is best described as an aromatic compound, not an alkene; that ring of unsaturation is not best described in terms of isolated 'pinned-down' olefins. Note also that the halogen substitution reactions that benzene undergoes is a totally different process from ones alkanes (or even simple alkenes) do. Reaction mechanisms are completely different -- no connection at all! However, having said that, if one HAS to choose between alkene or alkane, then definitely the answer would be alkene, since an alkane by strict definition would have no unsaturation at all.
Answers:A dimer forms, in order to decrease the dipole moment of the acid to zero, making it much more soluble in benzene. The two molecules join by two hydrogen bonds, carboxylate group to carboxylate group.