Electronic structure, amide group

The secondary amide group, -C(0)NHR, is ubiquitous in protein structures where hydrogen bonding from the amide NH group to the carbonyl oxygen atom is responsible for much of the secondary structure of proteins such as a-helices and (I-sheets. The electron withdrawing effect of the carbonyl oxygen... 

An understanding of the internal rotation about the amide bond is important because of its relevance to protein structure. Formamide is the simplest amide. The coplanarity and the remarkable rotational barrier about the C-N bond in formamide can be rationalized by resonance between the n electrons of the carbonyl group and the lone pair of the nitrogen atom [1, 50]. According to VB theory, the Jt electronic structure of formamide may be described by six resonance structures. 

The C—O double bond is the most important functional group in organic chemistry. It is present in aldehydes, ketones, acids, esters, amides, and so on. We shall spend Chapters 5-10 discussing its chemistry so it is important that you understand its electronic structure. As in alkenes, the two atoms that make up this double bond are sp2 hybridized. The carbon atom uses all three sp2 orbitals for overlap with other orbitals to form o bonds, blit the oxygen uses only one for overlap with another orbital (the sp2 orbitals on the carbon atom) to form a O bond. However, the other two sp2 orbitals are not vacant—they contain the oxygen s two lone pairs. A p orbital from the carbon and one from the oxygen make up the n bond which also contains two electrons. 

How do we know the amide group is planar X-ray crystal structures are the simplest answer. Other techniques such as electron diffraction also show that simple (noncrystalline) amides have planar structures. N,N- dimethyl for mamide (DMF) is an example. 

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