An allylic hydrogen is a hydrogen atom bonded to a carbon that sits directly next to a carbon-carbon double bond. It’s not part of the double bond itself, but it’s one bond away from it. That proximity gives allylic hydrogens distinctive chemical behavior: they’re easier to remove than most other hydrogens on a carbon chain, and they play important roles in both laboratory reactions and biological processes like fat oxidation.
Where Allylic Hydrogens Sit in a Molecule
To find allylic hydrogens, start by locating the carbon-carbon double bond in a molecule. The two carbons forming that double bond are called vinylic carbons, and any hydrogens directly attached to them are vinylic hydrogens. Now look one step further: the carbons bonded to those vinylic carbons (but not part of the double bond) are allylic carbons. Any hydrogen attached to an allylic carbon is an allylic hydrogen.
Propene (CH₂=CH-CH₃) is the simplest example. The first two carbons form the double bond, making them vinylic. The third carbon, the methyl group (CH₃), is bonded to a vinylic carbon but isn’t part of the double bond. Its three hydrogens are all allylic. In cyclohexene, the two carbons immediately next to the double bond are allylic, and the hydrogens on those carbons are allylic hydrogens.
The distinction matters because vinylic and allylic hydrogens behave very differently in reactions. Vinylic C-H bonds are among the strongest in organic chemistry, while allylic C-H bonds are among the weakest.
Why Allylic Hydrogens Are Easier to Remove
Breaking a chemical bond requires energy, and the amount needed varies depending on the bond’s environment. An allylic C-H bond requires only about 88 kcal/mol to break, compared to roughly 98 kcal/mol for a typical alkyl C-H bond (like those in ethane) and about 111 kcal/mol for a vinylic C-H bond. That makes the allylic position the easiest target when a reactive species comes looking for a hydrogen to grab.
The reason traces back to what happens after the hydrogen leaves. When an allylic hydrogen is removed, the carbon left behind forms either a radical (with an unpaired electron) or a charged species. That carbon can adopt a flat geometry, placing its lone electron or empty orbital in a p orbital that lines up with the p orbitals of the neighboring double bond. This overlap lets the electron (or charge) spread across three carbon atoms instead of staying stuck on one. Chemists call this resonance stabilization.
In the case of an allyl radical, the unpaired electron is delocalized across the entire three-carbon system. The true structure isn’t one with the radical on the left or right; it’s a blend of both. This spreading of electron density lowers the energy of the system, which is why forming an allylic radical costs less energy than forming a radical at a position farther from the double bond.
Allylic Hydrogens Are Slightly More Acidic
Acidity in organic chemistry refers to how readily a compound gives up a hydrogen as a proton. Allylic hydrogens have a pKa around 43, while standard alkane C-H bonds sit above 50. Lower pKa means higher acidity, so allylic hydrogens are roughly 10 million times more acidic than ordinary alkane hydrogens. That’s still extremely weak by everyday acid standards (water has a pKa of about 15.7), but in organic reactions where strong bases are used, the difference between pKa 43 and pKa 50+ determines which hydrogen gets pulled off first.
The same resonance stabilization explains this acidity. When an allylic hydrogen leaves as a proton, the resulting negative charge spreads over the three-carbon pi system, making the loss more energetically favorable than it would be at an isolated carbon.
Allylic Bromination With NBS
One of the most common reactions targeting allylic hydrogens is allylic bromination using N-bromosuccinimide, or NBS. This reaction selectively replaces an allylic hydrogen with a bromine atom while leaving the double bond intact. The selectivity comes from the bond energy differences: the allylic C-H bond is the weakest link in the molecule, so a bromine radical preferentially attacks there.
NBS works by maintaining a very low, steady concentration of bromine radicals in the reaction mixture. This is important because high concentrations of bromine would instead add across the double bond, destroying it. By keeping levels low and running the reaction in the presence of light or a radical initiator, the conditions favor substitution at the allylic position over addition to the double bond. The reaction is widely used in synthesis to introduce a bromine atom next to a double bond, which then serves as a handle for further chemical transformations.
Role in Lipid Peroxidation
Allylic hydrogens aren’t just a textbook concept. They play a central role in lipid peroxidation, a process that damages cell membranes and contributes to aging and disease. Polyunsaturated fatty acids, the fats found in cell membranes and in foods like fish and vegetable oils, contain multiple double bonds. The carbons sitting between two double bonds carry especially reactive allylic hydrogens (technically called bis-allylic, since they’re adjacent to two double bonds).
During lipid peroxidation, reactive oxygen species like hydroxyl radicals abstract an allylic hydrogen from a fatty acid chain. This creates a carbon-centered radical on the fat molecule. That radical rearranges to form a more stable structure called a conjugated diene, then reacts with oxygen to produce a lipid peroxyl radical. The peroxyl radical can then steal an allylic hydrogen from a neighboring fatty acid, starting a chain reaction. This propagation step is why a single radical event can damage many lipid molecules.
The consequences are significant. As fatty acid chains accumulate damage, the cell membrane loses its normal fluidity and structural integrity. Membrane-bound proteins stop functioning properly. The breakdown products of peroxidized lipids, including reactive aldehydes, cause further damage to proteins and DNA. This entire cascade begins with the vulnerability of allylic hydrogens to radical attack, a direct consequence of the low bond energy that makes them easy to remove.
Quick Comparison: Allylic vs. Vinylic vs. Alkyl
- Allylic hydrogens are on the carbon next to a double bond. Bond energy around 88 kcal/mol. Easiest to remove. The resulting radical or ion is stabilized by resonance with the adjacent pi system.
- Vinylic hydrogens are directly on a double-bond carbon. Bond energy around 111 kcal/mol. Hardest to remove of the three types, because the carbon uses a stronger sp² orbital to hold the hydrogen.
- Alkyl hydrogens are on carbons with no nearby double bond. Bond energy around 98-105 kcal/mol depending on whether the carbon is primary, secondary, or tertiary. No resonance stabilization available for the resulting radical.
This hierarchy explains the selectivity seen in many organic reactions. Whenever a reactive species has a choice of which hydrogen to abstract, allylic positions win out because they require the least energy and produce the most stable intermediates.