356
combustion with this high blend but found incompatibility of the fuel with high
butyl levulinate blend with the hoses and sealed engine fuel system. They have also
studied a 10% ethyl levulinate blend with diesel fuel and observed reduction in the
particulate matter emission up to 50% [59].The blending octane numbers for methyl
levulinate, ethyl levulinate, and iso-/secondary butyl levulinate were found to be
106.5, 107.5, and 102.5 [54]. The internal molecule organization, specifically
hydrogen bond network, makes alkyl levulinates more compressible. Esters derived
from levulinic acid possess very low vapor pressures as compared to the most commonly utilized solvent. Chlorinated solvents show more than 400 kPa vapor pressure, at 100 °C, while methyl, ethyl, and butyl levulinates show very less, i.e., 5.96,
4.69, and 1.56 kPa vapor pressure, respectively. Due to such kind of properties,
Bayarri Ferrer et al. have claimed the degreasing efficiency of alkyl levulinates
equivalent to the industrially utilized harmful trichloroethylene [60]. They have also
reported the VOC classification, cytotoxicity, and mutagenicity data for butyl and
pentyllevulinates. According to the reported data, they have confirmed the potential
of alkyl levulinate to be utilized as the green solvent [61].
4 Esters and Esterification Reaction: Chemistry
Esters are naturally occurring and vital organic compounds. They are responsible
for the fragrant odor of flowers and many fruits (e.g., methyl butanoate in pineapples, isopentyl acetate in banana oil). These pleasant smelling liquids are constitutional parts of animal fats and biologically significant molecules. Esters are
commonly made by a nucleophilic acyl substitution reaction between a carboxylic
acid and an alcohol. It involves initial protonation of carboxylic acid followed by
nucleophilic attack of ROH, an excess of the latter normally being employed to get
ester product as it facilitates shifting of equilibrium in the desired direction. Esters
result from simply heating a carboxylic acid in methanol or ethanol solution containing a small amount of mineral acid catalyst, as per discovery made by Fischer
and Speier in 1895 [62]. In this nucleophilic acyl substitution reaction, free carboxylic acids are hard to be attacked by most nucleophiles, but use of strong mineral
acid such as HCl and H 2 SO 4 boosts its reactivity. The mineral acid facilitates protonation of oxygen atoms of the carbonyl group, thereby generating positive charge
on carboxylic acid, thus making it much more reactive toward nucleophilic attack
by alcohol to yield tetrahedral intermediate. Transfer of proton from one oxygen to
another generates second tetrahedral intermediate and converts the –OH group into
a good leaving group. In the final step, protonated ester is formed by subsequent loss
of water, and the final deprotonation step leads to regeneration of acid catalyst to
form free ester product. Overall, Fischer esterification is nothing but is simply a
substitution of an –OH group by –OR′. The reaction steps involved are all reversible, and one can get either ester or carboxylic acid based on proper selection of
reaction conditions. The use of excess alcohol leads to ester formation, whereas the
use of excess water leads to carboxylic acid formation via acidic hydrolysis (initial
protonation of ester being followed by nucleophilic attack by water).
K. C. Maheria et al.
combustion with this high blend but found incompatibility of the fuel with high
butyl levulinate blend with the hoses and sealed engine fuel system. They have also
studied a 10% ethyl levulinate blend with diesel fuel and observed reduction in the
particulate matter emission up to 50% [59].The blending octane numbers for methyl
levulinate, ethyl levulinate, and iso-/secondary butyl levulinate were found to be
106.5, 107.5, and 102.5 [54]. The internal molecule organization, specifically
hydrogen bond network, makes alkyl levulinates more compressible. Esters derived
from levulinic acid possess very low vapor pressures as compared to the most commonly utilized solvent. Chlorinated solvents show more than 400 kPa vapor pressure, at 100 °C, while methyl, ethyl, and butyl levulinates show very less, i.e., 5.96,
4.69, and 1.56 kPa vapor pressure, respectively. Due to such kind of properties,
Bayarri Ferrer et al. have claimed the degreasing efficiency of alkyl levulinates
equivalent to the industrially utilized harmful trichloroethylene [60]. They have also
reported the VOC classification, cytotoxicity, and mutagenicity data for butyl and
pentyllevulinates. According to the reported data, they have confirmed the potential
of alkyl levulinate to be utilized as the green solvent [61].
4 Esters and Esterification Reaction: Chemistry
Esters are naturally occurring and vital organic compounds. They are responsible
for the fragrant odor of flowers and many fruits (e.g., methyl butanoate in pineapples, isopentyl acetate in banana oil). These pleasant smelling liquids are constitutional parts of animal fats and biologically significant molecules. Esters are
commonly made by a nucleophilic acyl substitution reaction between a carboxylic
acid and an alcohol. It involves initial protonation of carboxylic acid followed by
nucleophilic attack of ROH, an excess of the latter normally being employed to get
ester product as it facilitates shifting of equilibrium in the desired direction. Esters
result from simply heating a carboxylic acid in methanol or ethanol solution containing a small amount of mineral acid catalyst, as per discovery made by Fischer
and Speier in 1895 [62]. In this nucleophilic acyl substitution reaction, free carboxylic acids are hard to be attacked by most nucleophiles, but use of strong mineral
acid such as HCl and H 2 SO 4 boosts its reactivity. The mineral acid facilitates protonation of oxygen atoms of the carbonyl group, thereby generating positive charge
on carboxylic acid, thus making it much more reactive toward nucleophilic attack
by alcohol to yield tetrahedral intermediate. Transfer of proton from one oxygen to
another generates second tetrahedral intermediate and converts the –OH group into
a good leaving group. In the final step, protonated ester is formed by subsequent loss
of water, and the final deprotonation step leads to regeneration of acid catalyst to
form free ester product. Overall, Fischer esterification is nothing but is simply a
substitution of an –OH group by –OR′. The reaction steps involved are all reversible, and one can get either ester or carboxylic acid based on proper selection of
reaction conditions. The use of excess alcohol leads to ester formation, whereas the
use of excess water leads to carboxylic acid formation via acidic hydrolysis (initial
protonation of ester being followed by nucleophilic attack by water).
K. C. Maheria et al.