page_title: A value
section_title: Utility
hierarchical_section_title: A value / Utility
caption_attribution_description: English: A methyl substituent has a significantly smaller A-value than a tert-butyl substituent; therefore the most stable conformation has the tert-butyl in the equatorial position.
context_page_description: A-Values are numerical values used in the determination of the most stable orientation of atoms in a molecule, as well as a general representation of steric bulk. A-values are derived from energy measurements of the different cyclohexane conformations of a monosubstituted cyclohexane chemical.
  Substituents on a cyclohexane ring prefer to reside in the equatorial position to the axial. The difference in Gibbs free energy between the higher energy conformation and the lower energy conformation is the A-value for that particular substituent.
context_section_description: A-values help predict the conformation of cyclohexane rings.  The most stable conformation will be the one which has the substituent or substituents equatorial.  When multiple substituents are taken into consideration, the conformation where the substituent with the largest A-value is equatorial is favored.
  The utility of A-values can be generalized for use outside of cyclohexane conformations.  A-values can help predict the steric effect of a substituent.  In general, the larger a substituent's A-value, the larger the steric effect of that substituent.  A methyl group has an A-value of 1.74 while tert-butyl group has an A-value of ~5.  Because the A-value of tert-butyl is higher, tert-butyl has a larger steric effect than methyl.  This difference in steric effects can be used to help predict reactivity in chemical reactions.
caption: A methyl substituent has a significantly smaller A-value than a tert-butyl substituent; therefore the most stable conformation has the tert-butyl in the equatorial position.
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