Fatty Alcohols: Anthropogenic and Natural Occurrence in the Environment

by Stephen M Mudge, Scott E Belanger, Paul C DeLeo

Overview

Fatty alcohols are mainly used in the production of detergents and surfactants. They are also components of cosmetics, foods and as industrial solvents.

The 2008 edition originated from a multi-year review of the occurrence, behaviour and use of fatty alcohols to inform the registration of such chemicals through the REACH process in the EU. This review highlighted areas requiring further research. In this expanded edition, new information regarding products containing fatty alcohol derivatives, the fate of these down-the-drain products in wastewater systems and the use of compound specific stable isotope methods has been published. This work has been amalgamated with a number of aspects relating to the inclusion of these compounds in the EU bio-based economy drive.

Significant advances have been made since the first edition. No other book brings together all the disparate information regarding this group of chemicals that are of great interest to environmental scientist (as biomarkers), industry (as surfactants) and to regulators.

Product Details

ISBN-13:	9781788014212
Publisher:	Royal Society of Chemistry
Publication date:	09/28/2018
Sold by:	Barnes & Noble
Format:	eBook
Pages:	202
File size:	14 MB
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About the Author

Stephen M Mudge is currently at Bangor University, UK. After nearly six years of investigating radionuclides in the environment at Lancaster University, Stephen Mudge moved to Bangor Universityand began work on lipid biomarkers in the environment. In the past 20 years he has investigated the sources and dispersal of many contaminants, especially sewage derived materials, in marine and terrestrial environments. In 2003 he started the world's first degree on Environmental Forensics that utilises many of the chemical and statistical approaches developed over the years of investigation. Fatty alcohols have formed part of these analyses and while these compounds may be frequently measured, they are rarely reported. Scott Belanger is a Research Fellow in The Procter & Gamble Company corporate environmental safety organization. His research spans a wide range of topics including understanding the effects of consumer product chemicals in the environment at the levels of the organism to the ecosystem. He has assisted in several efforts to assess the environmental risk of alcohols and alcohol-derived surfactants in recent years frequently working with trade associations, academic partnerships, and the regulatory community on these affairs. Allen Nielsen is a recently retired microbiologist from the Research and Development Department of Sasol North America, Inc. His main focus during his thirty-one year career has been the environmental safety of petrochemical -derived surfactants which are used in consumer and industrial applications. In recent years he was focused on the environmental safety of alcohols and alcohol-derived surfactants.

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Fatty Alcohols

Anthropogenic and Natural Occurrence in the Environment

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By Stephen M Mudge, Scott E Belanger, Allen M Nielsen

The Royal Society of Chemistry

Copyright © 2008 ERASM
All rights reserved.
ISBN: 978-1-84755-859-6

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CHAPTER 1

Definitions

This chapter aims to introduce the family of compounds, how they are referred to, the likely structures that will be found and their chemistry from an environmental point of view.

1.1 Names and Structures

Fatty alcohol is a generic term for a range of aliphatic hydrocarbons containing a hydroxyl group, usually in the terminal or n-position. The accepted definition of fatty alcohols states that they are naturally derived from plant or animal oils and fats and used in the pharmaceutical, detergent or plastics industries (e.g. Dorland's Illustrated Medical Dictionary). However, it is possible to find the hydroxyl (-OH) group in other positions within the aliphatic chain, but these secondary or tertiary alcohols are not discussed to any great extent in this book.

The generic structure of fatty alcohols or n-alkanols can be seen in Figure 1.1 and specific examples in Figure 1.2. The value of the n-component is variable and is discussed below.

The range of chain lengths for these n-alkanols can be from 8 to values in excess of 32 carbons. With such a wide range of chain lengths, the chemical properties and consequently the environmental behaviour vary considerably. As well as these straight chain moieties, a range of branched chain compounds are also naturally produced by micro-organisms in the environment. The major positions for the methyl branches are on the carbons at the opposite end of the molecule to the terminal -OH. If the methyl branch is one position in from the end of the molecule (ω-1), it is termed an iso fatty alcohol; if it is two in from the end (ω-2), it is called an anteiso fatty alcohol. Examples of these branches can be seen in Figure 1.2.

Most fatty alcohols are saturated in that they have no double bonds present in their structure. However, there are a limited number of mono-unsaturated compounds that can be found in nature. The two most common compounds are phytol (3,7,11, 15-tetramethyl-2-hexadecen-1-ol), an isoprene derived from the side chain of chlorophyll (Figure 1.3), and a straight chain C20 alcohol with a double bond in the ω9 position counted from the terminal carbon (eicos-11-en-1-ol; Figure 1.3).

There have been occasional reports of polyunsaturated fatty alcohols, but these are relatively rare and are confined to di-unsaturates such as octadecadienol, 18:2. There is a group of isoprenoid lipids which may be found in bacteria and are essentially repeating isoprene subunits strung together and terminated by a hydroxyl group. These compounds are also uncommon in environmental analyses and are not reported to any great extent.

Fatty alcohols together with many other groups of compounds have both systematic and trivial or common names. The common name is based on the length of the alkyl chain and the root is common between aliphatic hydrocarbons and fatty acids. These common names together with the systematic name and carbon number are shown in Table 1.1.

1.2 Physicochemical Properties

1.2.1 Solubility Versus Chain Length

One of the key factors in determining the environmental behaviour of any compound is its water solubility; this will determine the partitioning between solid and solution phases. Compounds with low water solubility will be preferentially adsorbed to particulate matter, either settled or suspended in water. These compounds will also partition into the lipid phase of organisms and would have higher bioconcentration factors if not offset by metabolism. The available physicochemical properties for the fatty alcohol series from C4 to C30 are summarised in Table 1.1. These data are drawn from many sources, but principally from the online Beilstein Chemical Database (Elsevier MDL). The density and melting points in the summary data (Table 1.1) have a degree of uncertainty about them as some compounds, especially the longer chain and odd carbon number moieties, are less well studied. Density data are not available for all compounds.

The short chain compounds (up to C9) have appreciable water solubility (Table 1.1) and would not be classified as "fatty" alcohols as the free compounds are more likely to have a substantial amount in solution rather than in the solid phase (abiotic or biotic). Compounds with a chain length greater than 10 carbons are essentially insoluble in water and will partition on to the solid phase in the environment.

1.2.2 Partitioning (Kow) and Sediment Associations

It is usual to measure the water solubility and related factors such as bioconcentration factors (BCFs) through the octanol–water partition coefficient (Kow) or its logarithm (log Kow). There is relatively little information published for measured Kow values for fatty alcohols, although there are some data estimated from HPLC retention times. Difficulties arise in the measurement of these coefficients due to the hydrophobic–hydrophilic nature of the different parts of the molecule (Figure 1.4). The hydroxyl group gives that end of the molecule a degree of water solubility while the alkyl carbon chain is hydrophobic. Therefore, these compounds sit at the interface of octanol and water in the experimental situation.

The log Kow values for compounds (Table 1.2; shown graphically in Figure 1.5) with a chain length greater than C9 are above 4, which is indicative of materials that will be preferentially absorbed to particulate matter. In most environmental situations, this means the compounds will be associated mainly with particles such as settled and suspended sediments. The nature of these particulate materials is that they will settle out to the benthos at some stage and will be transferred to the geosphere. This partitioning between the solution phase for short chain compounds and solid phase for long chain compounds may lead to the separation of mixtures such that short chain moieties will remain in solution while longer chain moieties may settle out. There will also be different degradation steps possible as materials in the solid phase may enter anaerobic environments in sediments; this may lead to preservation of some materials and differential products of degradation.

The association of fatty alcohols with suspended matter will be of importance in sewage treatment plants as incoming materials may be removed from the system by partitioning into the solid phase which subsequently settles out. Experiments using radiolabelled alcohols with activated sewage sludge measured the time dependent partition coefficients (Kd) for a range of alcohols typically used in detergent formulations (Table 1.3). The mean (Kd) values can be seen in Figure 1.6; the data are presented on a logarithmic axis and a linear relationship can be seen in this figure. These values are relatively high implying that, in such a system, free fatty alcohols will be actively scavenged by the particulate phase and may be removed with the sludge or associated with suspended solids in wastewater. Thus, alcohols may leave sewage treatment plants either bound or unbound (free). Fisk et al. using the wastewater treatment model SIMPLETREAT (a module in the European environmental distribution model EUSES) demonstrated that as carbon number increases, the fraction of fatty alcohol that is degraded by microbes in wastewater declines as the amount sorbed increases. Federle and Itrich postulated that eventually at chain lengths of 16 and greater, the equilibrium desorption controls the biodegradation rate.

Summary

• Fatty alcohols found in the environment are principally linear with a terminal hydroxyl group.

• As the alkyl chain becomes longer, the water solubility decreases leading to a wide range of octanol-water partition coefficients.

• Compounds with a chain length greater than C10 are more likely to be associated with the solid phase in the aquatic environment and become coupled with sediments and soils in the environment.

• These lower water solubility compounds will tend to bioaccumulate more than their short chain moieties, although all may be metabolised by bacteria.

CHAPTER 2

Biological Synthesis

The biochemical mechanisms that lead to fatty alcohol formation highlight the differences between bacteria that can produce odd chain and branched compounds, while most other biota produce even chain compounds.

The synthesis of fatty alcohols by living organisms is intimately linked to the production of fatty acids in most cases. In order to understand the types of fatty alcohols present in the environment, it is necessary to appreciate the biochemical synthetic pathways that lead to their formation in the first place.

The formation of fatty acids can progress through two major pathways. Animals, fungi and some mycobacteria use the Type I synthetic pathway. In this pathway, the synthesis takes place within a large single protein unit and has a single product in the form of a C16 unsaturated fatty acid (palmitic acid). This system has genetic coding in one location. In contrast, plants and most bacteria use a series of small discrete proteins to catalyse individual steps within the synthesis, which is termed Type II fatty acid synthesis. These proteins are genetically encoded in several different locations. Yeasts are intermediate between these two extremes, where the synthesis activities take place in two separate polypeptides.

2.1 Type I Fatty Acid Synthesis

Type I fatty acid synthesis (FAS) occurs in animals. As well as having this initial style of fatty acid synthesis, there are a series of subsequent reactions which lead to the elongation of the primary fatty acid (hexadecanoic acid, C16) to higher carbon numbers and desaturation mechanisms leading to mono-unsaturated products. However, animals are unable to manufacture all the fatty acids they require and these must be obtained from plants in the diet (e.g. ω3 essential fatty acids).

The synthesis of fatty acids in this system occurs on a single large complex comprised of seven polypeptides. This complex acts as the focus for a series of reactions building the fatty acids up from an acetyl-CoA starter with malonyl-CoA subunits. The key components in the system can be seen in Figure 2.1. The complex performs four steps each time two carbons are added to the chain: initially CO2 is removed from the malonyl-CoA in a condensation reaction joining the two molecules together. NAD(P)H is used in a reduction step converting the C=O group to C–OH. This is dehydrated (removal of H2O) making a mid-chain double bond that undergoes a final reduction step with more NAD(P)H leading to a saturated alky chain.

The net effect of this series of four sub-reactions can be seen in Figure 2.2 as the product of the first step. The process is repeated until a 16-carbon chain has been created. The completed fatty acid is then cleaved from the FAS complex and is available for further reactions. This process explains why the most common fatty acid (and frequently fatty alcohol) found in environmental systems is comprised of 16 carbons. In some cases, an extra cycle occurs and a C18 fatty acid is formed instead.

2.1.1 Unsaturated Chains

In animals, fatty acyl-CoA desaturase catalyses the removal of two hydrogen atoms from the bond between C9 and C10 in either palmitic or stearic acid to provide the Δ9cis double bond in palmitoleic or oleic acid (Figure 2.3).

Table 2.1 shows the fatty acid content of several major oil and fat sources. In general, the animal sources have low quantities of unsaturated fatty acids while the plant sources have large amounts of polyunsaturated compounds. For example, for beef tallow the ratio of the saturated compounds to the unsaturated compounds is 0.9 compared to 15.7 for Canola or rapeseed oil. The exceptions to this are the oils derived from coconuts or palms which have large quantities of short chain unsaturated fatty acids making them amenable as a feedstock in some industrial processes (see Chapter 4).

2.2 Type II Fatty Acid Synthesis

Type II FAS in bacteria and plants occurs in a similar fashion to Type I, but the seven different polypeptides are independent of one another. The reactions are similar to those discussed above, but the products may then undergo a wider range of elongation and desaturation reactions. In the case of some plants (e.g. coconuts and palms; Figure 2.4), the fatty acid may be cleaved before it reaches 16 carbons and up to 90% of the oil from these plants may have fatty acids between C8 and C14.

2.2.1 Unsaturated Compounds

Unlike most animals, plants can introduce double bonds into fatty acids at locations other than the Δ9 position. They have enzymes that act on the Δ12 and Δ15 positions of oleic acid (18:1ω9) but only when it is part of a phospholipid or phosphatidylcholine. This specificity may explain why very few poly-unsaturated fatty alcohols are found.

Plants frequently contain fatty acids with two or more double bonds within the molecule (Table 2.1). For example, the principal fatty acid within linseed oil is linolenic acid or 18:3ω3, an 18-carbon straight chain molecule with three double bonds, the first of which is in position three from the ω end of the molecule (Δ15). Animals cannot generally make these polyunsaturated compounds and must obtain them from their diet, hence their being referred to as essential fatty acids. Once in animals, however, they may be elongated to form a range of other biochemically active compounds such as prosta-glandins.

2.2.2 Branched Chains

Bacteria make branched chain fatty acids and alcohols; the orientation of the carbons in the starter complex during the initial stages of FAS determines the final structure. There are three possible orientations which yield either a straight chained odd carbon numbered compound or two branched compounds with the methyl group in the iso or anteiso position (Figures 1.2 and 2.5).

It is also possible to start the fatty acid synthesis with an amino acid. The structure of the most appropriate molecules, valine and isoleucine, are shown in Figure 2.1. When valine is used, an iso-branched product of the FAS is formed while iso leucine yields anteiso-branched products. These compounds are the principal fatty acids in Gram-positive bacteria. The chain elongation process is the same as other higher organisms (e.g. Figure 2.2) but the starter compounds are different. Mid-chain branches are derived from other pathways where typically a methyl subunit is added across the double bond of an unsaturated compound such as oleic acid.

2.3 Fatty Acid Degradation

The C16 fatty acid produced by the Type I and Type II FAS pathways may undergo chain shortening (Figure 2.6) as well as elongation and desaturation. This is particularly important with regard to the formation of some fatty alcohols (see below) that require an appropriate fatty acid to start with. There are several enzyme systems involved in the process belonging to the acyl-CoA dehydrogenase family: those fatty acids with carbon chains between C12 and C18 use a long chain acyl-CoA dehydrogenase, a medium chain one operates on C4 to C14 acids while a short chain one acts on C4 to C6 only.

2.4 Fatty Acyl-CoA Reductase (FAR)

Fatty alcohols have several uses within an organism; they are principally associated with waxes and storage lipids although ether lipids also contain alcohols. Waxes are abundant neutral lipids that coat the surfaces of plants, insects and mammals. They are composed of long chain alcohols linked via an ester bridge to fatty acids and have the chemical property of being solid at room temperature and liquid at higher temperatures. Waxes have several essential biological roles including preventing water loss, abrasion and infection.

According to Cheng and Russell, who studied the synthesis of wax in mammals, two catalytic steps are required to produce a wax monoester (Figure 2.7). These include a reduction step of a fatty acid to a fatty alcohol and subsequently the transesterification of the fatty alcohol to a fatty acid. The first step is catalysed by the enzyme fatty acyl-CoA reductase (FAR) which uses the reducing equivalents of NAD(P)H to convert a fatty acyl-CoA into a fatty alcohol and Co-ASH. These enzymes must exist in several organisms as cDNAs specifying fatty acyl-CoA reductases have been identified in the jojoba plant, the silkworm moth, wheat and a micro-organism.

Fatty alcohols have two metabolic fates in mammals: incorporation into ether lipids or incorporation into waxes. Ether lipids account for ~20% of phospholipids in the human body and are synthesised in membranes by a pathway involving at least seven enzymes. The second step of this pathway is catalysed by the enzyme alkyl-dihydroxyacetone phosphate synthase, which exchanges a fatty acid in ester linkage to dihydroxyacetone phosphate with a long chain fatty alcohol to form an alkyl ether intermediate. Once produced, ether lipids are precursors for platelet activating factor, for cannabinoid receptor ligands and for essential membrane components in cells of the reproductive and nervous systems.

(Continues...)

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Excerpted from Fatty Alcohols by Stephen M Mudge, Scott E Belanger, Allen M Nielsen. Copyright © 2008 ERASM. Excerpted by permission of The Royal Society of Chemistry.
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Table of Contents

Definitions; Biological Synthesis; Occurrence in Biota; Consumer and Cosmetic Product Uses and Production; Environmental Transformations; Analytical Methods; Environmental Concentrations; Using Fatty Alcohols as Biomarkers; Multivariate Statistics; Environmental and Human Safety Aspects of Fatty Alcohols