What are the differences between primary, secondary, and tertiary substitution?

1 Answer
Truong-Son N.
Jan 18, 2018

Simply the number of substituents attached to the #alpha# carbon.

An example is ethoxide vs. tert-butoxide:

#1^@#, primary, indicates one substituent attached to the #alpha# carbon. #2^@#, secondary, indicates two attached, #3^@#, tertiary, indicates three attached.

  • The #"pK"_a# of ethanol is #15.9#, and the #"pK"_a# of tert-butanol is #16.5#. Ethanol is a stronger acid. The conjugate base of ethanol is therefore a weaker base, and so, tert-butoxide is the stronger base.
  • The faster nucleophile has less steric hindrance. So, ethoxide is the faster nucleophile, even though it is the weaker base.

Therefore, the #1^@# alkoxide is a better nucleophile and weaker base than the #3^@# alkoxide. Similarly, the #1^@# alcohol is the slower electrophile and stronger acid.

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