1.1- Dimethylethyl group

Dimethylethyl group (common name tert butyl)... 

SAMPLE SOLUTION (a) The molecule has a tert-butyl group bonded to a nine-membered cycloalkane. It is tert-butylcyclononane. Alternatively, the tert-butyl group could be named systematically as a 1,1-dimethylethyl group, and the compound would then be named (1,1-dimethylethyl)cyclononane. (Parentheses are used when necessary to avoid ambiguity. In this case the parentheses alert the reader that the locants 1,1 refer to substituents on the alkyl group and not to ring positions.) ... 

Conformational inversion of monosubstituted cyclohexanes results in equilibration of the substituent between axial and equatorial positions. The bulkier a substituent, the more it prefers the equatorial position. The 1.1-dimethylethyl group occupies the equatorial position exclusively,... 

Two more examples merit consideration. In 1,1-dimethylethylcyclo-hexane (25, -butylcyclohexane) the very bulky substituent group has an overwhelming preference for the equatorial position on account of its severe cis 1,3-diaxial interaction with two hydrogens when it is axial this can be verified with space-filling models. In effect, 25 is locked in the chair conformation with the 1,1-dimethylethyl group equatorial, incapable of inversion this substituent is discussed further in Chapter 6. 

In Summary The conformational analysis of cyclohexane enables us to predict the relative stability of its various conformers and even to approximate the energy differences between two chair conformations. Bulky substituents, particularly a 1,1-dimethylethyl group, tend to shift the chair-chair equilibrium toward the side in which the large substituent is equatorial. 

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