Hexenal, trans

A very large number of volatile substances have been identified in fresh tea leaf.64 Substances present at the highest levels include the ubiquitous leaf aldehyde, trans-2-hexenal, and leaf alcohol, cis-3-hexenol. Both arise from cis-3-hexenal, which is biosynthesized from linoleic acid in leaf as a result of enzymic splitting.65... 

The origin of many of the components of black tea aroma has been studied. Aldehydes are produced by catechin quinone oxidation of amino acids. Enzymic oxidation of carotenoids during manufacture generates ionones and their secondary oxidation products such as theaspirone and dihydroactinidolide. Oxidation of linoleic acid is responsible for the formation of trans-2-hexenal.82... 

Table 6.6, entry 3). In a competition experiment, trans-2-hexene was found to be hydrogenated much faster than its corresponding cis isomer (entry 4). 

Terminal RCH—CH2 1-Hexene C4H9CH=CH2 is isomerized by complex 1 in accordance with the factors influencing the thermodynamic stability of cis- and trans-2 -hexene [15], At the end of the reaction, the alkyne complex 1 was recovered almost quantitatively. No alkene complexes or coupling products were obtained. The corresponding zirconocene complex 2a did not show any isomerization activity. Propene CH3CH=CH2 reacts with complex 6 with substitution of the alkyne and the formation of zirconacydopentanes as coupling products, the structures of which are non-uniform [16]. 

HEXENE cis-2-HEXENE trans-2-HEXENE cis-3-HEXENE trans-3-HEXENE METHYLCYCLOPENTANE... 

Here are the key points. First, the most abundant volatiles in tomatoes are derived from catabolism of essential fatty acids. Linoleic acid is the precursor for hexanal and linolenic acid is that for cw-3-hexenal, cH-3-hexenol, and trans-2-hexenal. All of these are important flavor elements in the tomato. A healthy diet for people requires... 

Solid phase microextraction (SPME) is an ideal approach to monitor volatile flavor components. This approach has been used to identify the volatile compounds in the headspace of fresh fruit during maturation [92], Using SPME fibers and GC/MS, the key flavor components are hexanal, 2-isobutyl-3-methoxypyrazine, 2,3-butanedione, 3-carene, trans-2-hexenal, and linalool (Fig. 8.1). In this study, the principal aroma compounds whose abundance varied during fruit development were specifically identified. 

Additional aldehydes and ketones were also included in the U.S. Nationwide Occurrence Study dimethylglyoxal (2,3-butanedione), cyanoformaldehyde, 2-butanone (methyl ethyl ketone), trans-2-hexanal, 5-keto-l-hexanal, and 6-hydroxy-2-hexanone [11, 13]. Dimethylglyoxal was the most consistently detected of these carbonyl compounds (up to 3.5 pg/L) and was found at higher levels in plants using ozone. Maximum levels of 0.3, 5.0, and 0.7 pg/L were observed for cyanoformaldehyde, 2-butanone, and trans-2-hexenal, respectively 6-hydroxy-2-hexanone and 5-keto-1 -hexanal were only detected in early stages of treatment, and not in finished waters. 

The antennal olfactory receptor system in several phytophagous insects is very sensitive in the detection of the green odour components. In the Colorado beetle Leptinotarsa decemlineata, the threshold of response for trans-2-hexen-1-ol is circa 10b molecules per ml of air (17). In comparison, at 760 mm Hg and 20 C, 1 ml of air contains about 1019 molecules. The insects tested i.e., the migratory locust Locusta migratoria, the carrot fly Psila rosae (18), the cereal aphid Sitobion avenae (19), the Colorado beetle L. decemlineata (17), Leptinotarsa... 

The sensitivity of the antennal olfactory receptor system differs even between Colorado beetle populations (see Figure 5). The beetles of the field population in Wageningen are relatively more sensitive for cis-3-hexenyl acetate when tested than those of the laboratory stock culture. Beetles of the Utah population are relatively less sensitive for trans-2-hexen-1-ol and trans-2-hexenal than the individuals of the field population in Wageningen, and those insects obtained from the laboratory stock culture. The functional significance of these differences for the geographic variation in host plant range of this insect species needs further elucidation (21,22). 

Figure 3. Components trapped from the air over cut potato leaves. Carbon traps were extracted with 60 fxL of CSt IftL was used for CC (detector FID). GC conditions WCOT Carbowax 20M column, 50 m long temperature programmed 70-150 °C. Key a, trans-2-hexenal b, cis-3-hexenyl acetate c, cis-3-hexen-l-ol and d, tians-2-hexen-l-ol (14).

Problem 8.15 Dehydrohalogenation of 3-bromohexane gives a mixture of c -2-hexene and trans-2-hexene. How can this mixture be converted to pure (o) cis-2-hexene (b) frons-2-hexene ... 

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