Alkylbenzene sulfonates are a class ofanionic surfactants, consisting of ahydrophilicsulfonate head-group and ahydrophobic alkylbenzene tail-group. Along withsodium laureth sulfate, they are one of the oldest and most widely usedsynthetic detergents and may be found in numerous personal-care products (soaps, shampoos, toothpaste etc.) and household-care products (laundry detergent, dishwashing liquid, spray cleaner etc.).^[1]They were introduced in the 1930s in the form of branched alkylbenzene sulfonates (BAS). However following environmental concerns these were replaced with linear alkylbenzene sulfonates (LAS) during the 1960s.^[2] Since then production has increased significantly from about one million tons in 1980, to around 3.5 million tons in 2016, making them most produced anionic surfactant aftersoaps.^{[citation needed]}

An example of a branched alkylbenzene sulfonate (BAS)

Extensive foamingFremont, California - 1972

Branched alkylbenzene sulfonates (BAS) were introduced in the early 1930s and saw significant growth from the late 1940s onwards,^[3] in early literature these synthetic detergents are often abbreviated as syndets. They were prepared by theFriedel–Crafts alkylation ofbenzene with 'propylene tetramer' (also called tetrapropylene) followed bysulfonation. Propylene tetramer being a broad term for a mixture of compounds formed by theoligomerization ofpropene, its use gave a mixture of highly branched structures.^[4]

Compared to traditional soaps, BAS offered superior tolerance tohard water and better foaming.^[5] However, the highly branched tail made it difficult tobiodegrade.^[6] BAS was widely blamed for the formation of large expanses of stable foam in areas of wastewater discharge such as lakes, rivers and coastal areas (sea foams), as well as foaming problems encountered in sewage treatment^[7] and contamination of drinking water.^[8] As such, BAS was phased out of most detergent products during the 1960s, being replaced with linear alkylbenzene sulfonates (LAS), which biodegrade much more rapidly. BAS is still important in certain agrochemical and industrial applications, where rapidbiodegradability is of reduced importance. For instance, inhibitingasphaltene deposition fromcrude oil.

Linear alkylbenzene sulfonates (LAS) are prepared industrially by thesulfonation oflinear alkylbenzenes (LABs), which can themselves be prepared in several ways.^[2] In the most common routebenzene is alkylated by long chain monoalkenes (e.g. dodecene) usinghydrogen fluoride as a catalyst.^[9] The purifieddodecylbenzenes (and related derivatives) are thensulfonated withsulfur trioxide to give thesulfonic acid.^[10] The sulfonic acid is subsequently neutralized withsodium hydroxide.^[1]The term "linear" refers to the startingalkenes rather than the final product, perfectly linear addition products are not seen, in-line withMarkovnikov's rule. Thus, the alkylation of linear alkenes, even1-alkenes such as1-dodecene, gives several isomers of phenyldodecane.^[11]

Under ideal conditions the cleaning power of BAS and LAS is very similar, however LAS performs slightly better in normal use conditions, due to it being less affected byhard water.^[12]Within LAS itself the detergency of the various isomers are fairly similar,^[13][14] however their physical properties (Krafft point, foaming etc.) are noticeably different.^[15][16]In particular the Krafft point of the high 2-phenyl product (i.e. the least branched isomer) remains below 0 °C up to 25% LAS whereas the low 2-phenylcloud point is ~15 °C.^[17] This behavior is often exploited by producers to create either clear or cloudy products.

The biodegradability of alkylbenzene sulfonates has been well studied,^[6][18][19] and is affected byisomerization, in this case, branching. The salt of the linear material has anLD₅₀ of 2.3 mg/liter for fish, about four times more toxic than the branched compound; however the linear compound biodegrades far more quickly, making it the safer choice over time. It is biodegraded rapidly under aerobic conditions with ahalf-life of approximately 1–3 weeks;^[18] oxidative degradation initiates at the alkyl chain.^[1] Under anaerobic conditions it degrades very slowly or not at all, causing it to exist in high concentrations insewage sludge, but this is not thought to be a cause for concern as it will rapidly degrade once returned to an oxygenated environment.

• Organic sodium salts
• Cleaning product components
• Anionic surfactants
• Sulfonates
• Glycine receptor agonists
• Alkyl-substituted benzenes

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