Some Facts about Double Bonds

A trigonal carbon atom is bonded to only three other atoms. 

Four models of ethylene The dash-wedge, ball-and-stick, and space-filling models show that the four atoms attached to a carbon-carbon double bond lie in a single plane. The electrostatic potential map shows the electron density (red) above and below the plane that passes through the carbon and hydrogen nuclei. 

These differences between single and double bonds are summarized in Table 3.1. Let us see how the orbital model for bonding can explain the structure and properties of double bonds. 

Number of atoms attached to a carbon 4 (tetrahedral) 3 (trigonal) 

Geometry many conformations are possible staggered is preferred planar 

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Some Facts about Double Bonds The Orbital Model of a Double Bond the Pi Bond Cis Trans Isomerism in Alkenes A WORD ABOUT... 

Scheme 6.13 gives some examples of Cope and oxy-Cope rearrangements. Entry 1 shows a reaction that was done to compare the energy of chair and boat TSs. The chiral diastereomer shown can react through a chair TS and has a AG about 8 kcal/mol lower than the meso isomer, which must react through a boat TS. The equilibrium is biased toward product by the fact that the double bonds in the product are more highly substituted, and therefore more stable, than those in the reactant. 

It is readily apparent that the three a bonds are capable of holding the six bonding electrons in the a t and e molecular orbitals. The possibility of some 7r-bonding is seen in the molecular orbital diagram as a result of the availability of the a2" orbital, and in fact there is some experimental evidence for this type of interaction. The sum of the covalent radii of boron and fluorine atoms is about 152 pm (1.52 A), but the experimental B-F bond distance in BF3 is about 129.5 pm (1.295 A). Part of this "bond shortening" may be due to partial double bonds resulting from the 7r-bonding. A way to show this is by means of the three resonance structures of the valence bond type that can be shown as... 

A protein s secondary structure arises from the formation of intra- and inter-molecular hydrogen bonds. All carboxyl group oxygens and amine hydrogens of a polypeptide participate in H-bonding. Protein secondary structure also derives from the fact that although all C-N bonds in peptides have some double bond character and cannot rotate, rotation about the Co-N and Ca-C bonds is possible and is... 

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