thumb|An example of a fatty alcohol, oleyl alcohol

Fatty alcohols (or long-chain alcohols) are usually high-molecular mass, straight-chain primary alcohols, but can also range from as few as 4–6 carbon atoms to as many as 22–26, derived from natural fats and oils. The precise chain length varies with the source. Some commercially important fatty alcohols are lauryl, stearyl, and oleyl alcohol. They are colourless oily liquids (for smaller carbon numbers) or waxy solids, although impure samples may appear yellow. Fatty alcohols usually have an even number of carbon atoms and a single alcohol group (–OH) attached to the terminal carbon. Some are unsaturated and some are branched. They are widely used in industry. As with fatty acids, they are often referred to generically by the number of carbon atoms in the molecule, such as "a C<sub>12</sub> alcohol", that is an alcohol having 12 carbon atoms, for example dodecanol.

Production and occurrence

Fatty alcohols became commercially available in the early 1900s. They were originally obtained by reduction of wax esters with sodium by the Bouveault–Blanc reduction process. In the 1930s catalytic hydrogenation was commercialized, which allowed the conversion of fatty acid esters, typically tallow, to alcohols. In the 1940s and 1950s, petrochemicals became an important source of chemicals, and Karl Ziegler had discovered the polymerization of ethylene. These two developments opened the way to synthetic fatty alcohols. As of 2005, about 50% of fatty alcohols used commercially are of natural origin, the remainder being synthetic (petrochemical). They are produced by bacteria, plants and animals for purposes of buoyancy, as source of metabolic water and energy, biosonar lenses (marine mammals) and for thermal insulation in the form of waxes (in plants and insects).

The traditional sources of fatty alcohols have largely been various vegetable oils, which remain a large-scale feedstock. Animal fats (tallow) were of historic importance, particularly whale oil, however they are no longer used on a large scale. Tallows produce a fairly narrow range of alcohols, predominantly C<sub>16</sub>–C<sub>18</sub>, while plant sources produce a wider range of alcohols (C<sub>6</sub>–C<sub>24</sub>), making them the preferred source. The alcohols are obtained from the triglycerides (fatty acid triesters), which form the bulk of the oil. The process involves the transesterification of the triglycerides to give methyl esters which are then hydrogenated to produce fatty alcohols. Higher alcohols (C<sub>20</sub>–C<sub>22</sub>) can be obtained from rapeseed oil or mustard seed oil. Midcut alcohols are obtained from coconut oil (C<sub>12</sub>–C<sub>14</sub>) or palm kernel oil (C<sub>16</sub>–C<sub>18</sub>).

From petrochemical sources

Fatty alcohols are also prepared from petrochemical sources. In the Ziegler process, ethylene is oligomerized using triethylaluminium followed by air oxidation. This process affords even-numbered alcohols:

:Al(C<sub>2</sub>H<sub>5</sub>)<sub>3</sub> + 18&nbsp;C<sub>2</sub>H<sub>4</sub> → Al(C<sub>14</sub>H<sub>29</sub>)<sub>3</sub>

:Al(C<sub>14</sub>H<sub>29</sub>)<sub>3</sub> + &nbsp;O<sub>2</sub> + &nbsp;H<sub>2</sub>O → 3&nbsp;HOC<sub>14</sub>H<sub>29</sub> + &nbsp;Al<sub>2</sub>O<sub>3</sub>

Alternatively ethylene can be oligomerized to give mixtures of alkenes, which are subjected to hydroformylation, this process affording odd-numbered aldehyde, which is subsequently hydrogenated. For example, from 1-decene, hydroformylation gives the C<sub>11</sub> alcohol:

:C<sub>8</sub>H<sub>17</sub>CH=CH<sub>2</sub> + H<sub>2</sub> + CO → C<sub>8</sub>H<sub>17</sub>CH<sub>2</sub>CH<sub>2</sub>CHO

:C<sub>8</sub>H<sub>17</sub>CH<sub>2</sub>CH<sub>2</sub>CHO + H<sub>2</sub> → C<sub>8</sub>H<sub>17</sub>CH<sub>2</sub>CH<sub>2</sub>CH<sub>2</sub>OH

In the Shell higher olefin process, the chain-length distribution in the initial mixture of alkene oligomers is adjusted so as to more closely match market demand. Shell does this by means of an intermediate metathesis reaction. The resultant mixture is fractionated and hydroformylated/hydrogenated in a subsequent step.

Applications

Fatty alcohols are mainly used in the production of detergents and surfactants. Being viscous and immiscible with water, they find use as co-emulsifiers, emollients, and thickeners in cosmetics and food industry.

:

The resulting fatty alcohol ethoxylates are important surfactants. They are named after the parent fatty alcohol (such as lauryl alcohol and stearyl alcohol) with an "eth" suffix, i.e., laureth and steareth. These terms are followed by an index for the average number of ethoxylate groups; for example, laureth-20 denotes an ethoxylated lauryl alcohol with about 20 ethoxy units on average and has the formula .

<!--Mostly red-links, possibly of future use:==Types==

  • Normal-chain alcohols
  • Saturated alcohols
  • Unsaturated alcohols
  • Acetylenic alcohols
  • Sulfated alcohols
  • Branched-chain alcohols
  • Mono-methylated alcohols
  • Polyisoprenoid alcohols
  • Saturated polyisoprenoids (Isopranols)
  • Unsaturated polyisoprenoids (prenols or polyprenols) incl turpenols.
  • Phenolic alcohols (aka phenolphthiocerol)-->

Safety

Human health

Fatty alcohols are relatively benign materials, with LD<sub>50</sub> (oral, rat) ranging from 3.1–4&nbsp;g/kg for hexanol to 6–8&nbsp;g/kg for octadecanol. Fatty alcohols exhibit no skin sensitization.

Repeated exposure to fatty alcohols produce low-level toxicity and certain compounds in this category can cause local irritation on contact or low-grade liver effects (essentially linear alcohols have a slightly higher rate of occurrence of these effects). No effects on the central nervous system have been seen with inhalation and oral exposure. Tests of repeated bolus dosages of 1-hexanol and 1-octanol showed potential for CNS depression and induced respiratory distress. No potential for peripheral neuropathy has been found. In rats, the no observed adverse effect level (NOAEL) ranges from 200&nbsp;mg/kg/day to 1000&nbsp;mg/kg/day by ingestion. There has been no evidence that fatty alcohols are mutagenic or cause reproductive toxicity or infertility. Fatty alcohols are effectively eliminated from the body when exposed, limiting possibility of retention or bioaccumulation.

Environment

Fatty alcohols up to chain length C<sub>18</sub> are biodegradable, with length up to C<sub>16</sub> biodegrading within 10 days completely. Chains C<sub>16</sub> to C<sub>18</sub> were found to biodegrade in rates ranging from 62% to 76% in 10 days. Chains greater than C<sub>18</sub> were found to degrade by 37% in 10 days. Field studies at wastewater treatment plants have shown that 99% of fatty alcohols of lengths C<sub>12</sub>–C<sub>18</sub> are removed.