2.2. Terminology – Alpha, Beta, Gamma vs. 2, 3, 4

Reactions involving carbonyl-derived nucleophiles occur near the carbonyl. Two notation systems to specify and identify these positions were developed.

Under the older convention positions are labelled alphabetically using lowercase Greek letters (Figure 2.3). The position adjacent to the carbonyl is alpha (α), then each further position away position is labelled in Greek alphabetical order (beta β, gamma γ, delta δ, epsilon ε, etc.).

Figure 2.3 – Greek Notation for Positions Near a Carbonyl.

In rare instances the site of the carbonyl itself may also labelled as the ipso position. If there are also positions of interest on the other side of the carbonyl, such as if the carbonyl is in an unsymmetrical ketone, then these are labelled in the same way but with a “prime” symbol to denote the difference: α’, β’, etc. (Figure 2.4). Both these modifications to the notation system will be avoided in this text but may be encountered in some sources.

Figure 2.4 – Extended Greek Notation for Positions Near a Carbonyl.

Under the newer convention positions are labelled numerically (Figure 2.5). The carbonyl is considered position 1 and positions are numbered as they extend from it.

Figure 2.5 – Numerical Notation for Positions Near a Carbonyl.

As before, the “prime” modifier may be used when describing positions of interest on the other side of the carbonyl.

Recent efforts have attempted to shift discussion to the modern numerical system. However, the overwhelming majority of chemists use the older Greek system especially when discussing the position adjacent to the carbonyl (alpha, α).