Chemistry of Henna

CHEMISTRY OF HENNA

The chemistry of henna (Lawsonia inermis) is very interesting - it is an amalgamation of organic chemistry, natural dyes, and skin chemistry. Let’s break it down simply

1. Source

The henna is derived out of the leaves of the plant Lawsonia inermis.

Lawsone is the active dye molecule of henna.

Chemical name: 2-hydroxy-1,4-naphthoquinone

Molecular formula: C₁₀H₆O₃

Form: It is a naphthoquinone analog of anthraquinone-type dyes.

2. Chemistry of Lawsone

Functional groups:

Hydroxyl group (-OH)

Two (carbonyl) C=O groups in quinone ring.

This renders lawsone a weak acid and provides the latter with the capacity to develop hydrogen bonds and react with proteins.

3. Mechanism /How Henna stains Skin/Hair

When henna paste is applied:

The molecules of lawsone permeate the epidermis or hair.

They attach themselves to keratin which is the major protein on the hair and skin.

It reacts by Michael addition - the quinone group in lawsone reacts with the amino groups ( -NH -) in keratin.

The stable covalent bond that is formed is what makes the reddish-brown color.

The stain is formed when the dye is oxidized and polymerized in the air, and it deepens.

4. Henna on Hair

Lawsone reacts with the hair keratin → semi-permanent orange to red.

With indigo powder (Indigofera tinctoria) it produces brown or black color, the lawsone combines with indigotin dye.

5. “Black Henna” Warning

Natural henna is reddish-orange in color only.

Black henna is frequently mixed with para-phenylenediamine (PPD) - an artificial dye that may produce allergic effects, burns or blisters.

6. Fun Fact

Lawsone is antibacterial and antioxidant, the reason why henna is used traditionally as a cooling, healing and protective agent to the skin.