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CH2 OH CH O H CH2 OH
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CHLOROPROPANOLS
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CH2 OH
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CH2 OH H Cl CH O H CH2 O H H Cl glycerol
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CH O H CH OH - H2O CH2 O H Cl CH2 OH2 CH2 Cl H Cl alkyloxonium ion SN2 transition state 3-MCPD
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CH2 OH
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H CH O H CH2 OH
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CH2 OH CH OH2 CH2 OH - H2O
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CH2 OH CH CH2 OH
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CH2 OH CH Cl CH2 OH 2-MCPD
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alkyloxonium ion SN1 transition state
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carbocation
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Reaction of glycerol with hydrochloric acid.
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can form either monochloroderivatives or dichloroderivatives. Glycerol reacts with HCl with the formation of 3-MCPD and 2-MCPD. The distribution of both these isomers (approximately present in the ratio of 2 : 1) is the result of a nucleophilic substitution of the hydroxyl groups by chloride anion, in accord with statistical substitution of two equivalent primary hydroxyls and one secondary hydroxyl group (44). Hydroxyl groups of glycerol are rst protonated by HCl to alkyloxonium ions (conjugated acids). With the primary hydroxyls, the next stage is an SN2 reaction in which the chloride ion displaces a molecule of water from the alkyloxonium cation. This pathway is stereospeci c and proceeds with inversion of con guration at the carbon that bears the leaving group (45). According to the theory, it yields a racemic mixture of both enantiomers of 3-MCPD. With the secondary hydroxyl group, this stage is an SN1 reaction in which the alkyloxonium ion dissociates to a carbocation and water. Following its formation, the carbocation is captured by chloride ion under the formation of 2-MCPD (Fig. 6.3.4). The ratio of 3-chloro isomer to 2-chloro isomer is about 2.3. The yield of chloropropanediols can be enhanced in the presence of carboxylic acids (such as acetic acid and to a much smaller extent by the addition of fatty acids or amino acids) that form with glycerol the corresponding esters (predominantly 1-acyl-sn-glycerol and to a smaller extent 2-acyl-sn-glycerol) in acid solutions (Fig. 6.3.5) (45). Monoacylglycerols readily eliminate hydroxyl groups yielding a cyclic acyloxonium ion intermediate. The reaction of the acyloxonium ion intermediate with chloride anion leads mostly to 2-ester of 3-MCPD. The corresponding 1-ester is a minor product. Both monoesters are hydrolyzed to 3-MCPD and 2-MCPD, respectively. For example, the ratio of MCPDs in the presence of acetic acid is higher than in the case of glycerol without acetic acid being about 6.4 as it is controlled by the steric and electronic effects arising from the terminal ester group, which directs substitution of chloride ion primarily to the CH2 carbon atom. Analogous reactions can
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CH2 OH CH OH R COOH O CH2 O C R CH OH CH2 OH - HO 1-acyl-sn-glycerol Cl CH2 Cl O CH O C R CH2 OH CH2 O CH O CH2 OH acyloxonium ion CH Cl CH2 OH 1-acyl-2-chloropropane-1,2-diol H2O - R COOH C R - HO Cl - H2O CH2 OH glycerol - H2O CH2 OH O CH O C R CH2 OH 2-acyl-sn-glycerol O CH2 O C R R COOH
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2-acyl-3-chloropropane-1,2-diol H2O - R COOH
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CH2 OH CH OH CH2 Cl 3-MCPD
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CH2 OH CH Cl CH2 OH 2-MCPD
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Figure 6.3.5 Formation of chloropropanediols from glycerol in the presence of acids.
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proceed with partial acylglycerols (monoacylglycerols and diacylglycerols) derived from higher fatty acids. Another way in which chloropropanediols can arise is a direct substitution of the acyl groups or hydroxyl groups in monoacylglycerols by chloride anions. Another possible mechanism for the formation of MCPDs from glycerol via intermediate glycidol can more likely proceed in low-moisture food (see 2.6). MCPDs react with another molecule of HCl forming DCPs. Hydroxyl groups of MCPDs are eliminated as water and the intermediate carbocations combine with chloride anions yielding DCPs (45). The proposed simpli ed reaction mechanism starting from 3-MCPD and 2-MCPD is given in Fig. 6.3.6. Similarly to the formation of 2-MCPD from glycerol (Fig. 6.3.4), the chlorination of 3-MCPD at the secondary hydroxyl group can also yield 2,3-DCP. 6.3.4.2 Formation from Triacylglycerols
A simpli ed reaction scheme leading to the formation of 3-MCPD from triacylglycerols is given in Fig. 6.3.7 (reactions leading to 2-MCPD are not given). Triacylglycerols are rst transformed to diacylglycerols by acid hydrolysis and
CH2 OH CH OH CH2 Cl 3-MCPD CH2 OH2 CH OH CH2 Cl - H2O CH2 CH OH CH2 Cl carbocation
CHLOROPROPANOLS
CH2 Cl CH OH CH2 Cl 1,3-DCP
alkyloxonium ion
CH2 OH CH Cl CH2 OH 2-MCPD
CH2 OH2 CH Cl CH2 OH - H2O
CH2 CH Cl CH2 OH carbocation
CH2 Cl CH Cl CH2 OH 2,3-DCP
alkyloxonium ion
Formation of dichloropropanols from chloropropanediols.
O O CH2 O C R O CH2 O C R 1,2,3-triacyl-sn-glycerol O R C O CH R C O CH CH2 Cl
O CH2 O C R
1,2-diacyl-3-chloropropane-1,2-diol
O CH2 O C R HO CH O CH2 O C R 1,3-diacyl-sn-glycerol O R C O CH CH2 OH
O CH2 O C R
O CH2 O C R HO CH CH2 Cl 1-acyl-3-chloropropane-1,2-diol
1,2-diacyl-sn-glycerol
O CH2 O C R HO CH CH2 OH 1-acyl-sn-glycerol O CH2 OH CH2 OH O CH2 OH CH2 Cl R C O CH R C O CH
2-acyl-sn-glycerol 2-acyl-3-chloropropane-1,2-diol
CH2 OH CH OH CH2 OH glycerol
CH2 OH CH OH CH2 Cl 3-MCPD
Formation of MCPD from triacylglycerols.
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hydrolysis of diacylglycerols then leads to monoacylglycerols and nally to glycerol (Fig. 6.3.6). The principal reaction leading to 3-MCPD diesters is explained, in accord with the theory, as proceeding via the partial diacylglycerols with the ester group(s) providing anchimeric assistance through the formation of an acylated cyclic acyloxonium ion intermediate (44), analogously to the reaction of glycerol with HCl in the presence of carboxylic acids (Fig. 6.3.5). Diacylglycerols can be also hydrolyzed to monoacylglycerols that can again form the cyclic acyloxonium ions, which are opened by chloride anions to yield 3-MCPD monoesters. 3-MCPD diesters can be hydrolyzed to 3-MCPD monoesters and these to 3-MCPD. Another pathway that can take place in the frame of this complex reaction is a direct substitution of either the acyl group (acidolysis by HCl) or hydroxyl group by chloride anions in acylglycerols. The ratio of chloropropanediols is again controlled by the steric and electronic effects arising from the terminal ester group, which directs substitution to the CH2 carbon atom. Additionally, the regiospeci city is greater with 3isomer to 2-isomer ratio of approximately 10 : 1 (44). 6.3.4.3 Formation from Phospholipids
Phospholipids are apparently hydrolyzed by HCl to totally deacylated derivatives, for example, (3-sn-phosphatidyl)choline (1,2-diacyl-sn-glycero-3-phosphocholine) yields sn-glycero-3-phosphocholine (Fig. 6.3.8). 1,2-Diacyl-sn-glycero-3phosphatidylcholine can react with chloride ions yielding, for example, the