= Tocotrienol side-chain unsaturation
\b[<Tocotrienol side-chain unsaturation>] is the presence of three carbon–carbon <double bonds> in the hydrocarbon chain attached to the chromanol head of a <tocotrienol>. The corresponding <tocopherol> chain is saturated. The nomenclature locates these bonds at $3'$, $7'$ and $11'$, with primes distinguishing side-chain numbering from ring numbering.<Nomenclature of Tocopherols and Related Compounds by IUPAC-IUB Joint Commission on Biochemical Nomenclature>[Recommendations 6 and 12d]{ref}
The positions matter as well as the count. An original unsaturation calculation compares <alpha-tocotrienol>, $\mathrm{C_{29}H_{44}O_2}$, with <alpha-tocopherol>, $\mathrm{C_{29}H_{50}O_2}$. Their <degree of unsaturation> values, calculated as $C+1-H/2$ for these carbon–hydrogen–oxygen formulas, are respectively 8 and 5. The difference of three matches the three additional <double bonds>. This calculation counts structural constraints; it does not identify where they occur.
Each side-chain <double bond> also constrains local rotation. The terminal $11'$ bond has two identical methyl substituents on one of its carbons, so it cannot independently receive an E/Z geometric label. The nomenclature's $(E,E)$ designation concerns the other two side-chain bonds.<Nomenclature of Tocopherols and Related Compounds by IUPAC-IUB Joint Commission on Biochemical Nomenclature>[Recommendation 6, parent structure]{ref}
Ideal <hydrogenation> removes those three additional unsaturations, but the product's <stereochemistry> requires separate accounting. Saturating the $3'$ and $7'$ bonds makes positions $4'$ and $8'$ new <stereocenters>. Holding an initial ring configuration fixed permits $2^2=4$ side-chain configurations. The nomenclature explicitly distinguishes this four-form product from <all-rac-alpha-tocopherol>.<Nomenclature of Tocopherols and Related Compounds by IUPAC-IUB Joint Commission on Biochemical Nomenclature>[Recommendation 12d]{ref} A mass or formula measurement can confirm an atom balance without establishing which configurations were obtained.
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