Alcohols

In short: Alcohols are organic compounds with an −OH group, like ethanol. Hydrogen bonding gives them high boiling points, primary alcohols oxidise to aldehydes and then carboxylic acids, secondary alcohols oxidise to ketones, and tertiary alcohols do not oxidise.

The hydroxyl group

An alcohol has an −OH (hydroxyl) group bonded to a carbon. The general formula of the alkanols is ${\mathrm{C}}_{\mathrm{n}}{\mathrm{H}}_{2n+1}\mathrm{OH}$.

Note: the −OH in an alcohol is not a hydroxide ion. Alcohols are neutral, not basic.

Primary, secondary and tertiary

Look at the carbon that holds the −OH. Count how many other carbons are bonded to it.

Physical properties

• Alcohols form hydrogen bonds (O−H). So they have much higher boiling points than alkanes of similar mass. Ethanol boils at 78 °C; propane (similar mass) at −42 °C.
• Small alcohols mix completely with water, because their −OH hydrogen bonds with water.
• As the carbon chain gets longer, the non-polar part grows, so solubility falls and boiling point rises.

Combustion

Alcohols burn cleanly with a blue flame:

$${\mathrm{C}}_2{\mathrm{H}}_5\mathrm{OH}(\mathrm{l})+3{\mathrm{O}}_2(\mathrm{g})\to 2C{\mathrm{O}}_2(\mathrm{g})+3{\mathrm{H}}_2\mathrm{O}(\mathrm{l})$$

The molar heat of combustion $\Delta H_c$ is the heat released when one mole burns completely in oxygen. It is always negative.

Measuring it with a spirit burner and a can of water:

1. Weigh the spirit burner.
2. Measure the water's mass and starting temperature.
3. Burn the alcohol under the can, stirring, until the temperature rises about 20 °C.
4. Reweigh the burner. The mass lost is the alcohol burnt.

$$q=mc\Delta Tn=\frac{\text{mass burnt}}{MM}\Delta H_c=-\frac{q}{n}$$

Lab values are low because of heat lost to the air and the can, incomplete combustion (soot on the can), and alcohol evaporating from the wick. Using a draught shield, a lid, and a copper can close to the flame helps.

As alcohols get longer, each extra $\mathrm{C}{\mathrm{H}}_2$ adds about 650 kJ/mol to $\Delta H_c$.

Reactions of alcohols

$$\mathrm{C}{\mathrm{H}}_3\mathrm{C}{\mathrm{H}}_2\mathrm{OH}\stackrel{\text{conc. }{\mathrm{H}}_2\mathrm{S}{\mathrm{O}}_4\text{, heat}}{\to }\mathrm{C}{\mathrm{H}}_2=\mathrm{C}{\mathrm{H}}_2+{\mathrm{H}}_2\mathrm{O}$$

Dehydration is the reverse of hydration.

Oxidation

To stop a primary alcohol at the aldehyde, distil it off as it forms. To go all the way to the carboxylic acid, heat under reflux.

This gives a test: tertiary alcohols do not change the colour of acidified dichromate.

Making ethanol

$${\mathrm{C}}_6{\mathrm{H}}_{12}{\mathrm{O}}_6(\mathrm{aq})\stackrel{\text{yeast}}{\to }2{\mathrm{C}}_2{\mathrm{H}}_5\mathrm{OH}(\mathrm{aq})+2C{\mathrm{O}}_2(\mathrm{g})$$

Fermentation needs no oxygen, otherwise yeast makes $\mathrm{C}{\mathrm{O}}_2$ and water instead. The mass of the flask drops as $\mathrm{C}{\mathrm{O}}_2$ escapes, which lets you track the reaction.

Biofuels

Ethanol made from plants is a biofuel.

• For: renewable; the $\mathrm{C}{\mathrm{O}}_2$ released was recently taken in by the growing plants; burns more cleanly; can be blended with petrol (E10).
• Against: less energy per gram than petrol (about 30 kJ/g compared with 47 kJ/g); uses farmland and water that could grow food; fertilisers, harvesting and distillation use fossil fuel energy; can absorb water and corrode engines.

Quick summary

• Alcohols have an −OH group. Primary, secondary or tertiary depends on carbons attached to the C−OH carbon.
• Hydrogen bonding gives high boiling points and water solubility for small alcohols.
• $\Delta H_c=-q/n$; lab values are low because of heat loss and incomplete combustion.
• Dehydration (conc. ${\mathrm{H}}_2\mathrm{S}{\mathrm{O}}_4$) gives alkenes. HX gives haloalkanes.
• Oxidation: 1° → aldehyde → acid; 2° → ketone; 3° no reaction (dichromate stays orange).
• Ethanol is made by fermentation (renewable, slow) or hydration of ethene (fast, pure, from oil).

Quiz