Synthesis of carboxylic acids

Most of the methods for the synthesis of carboxylic acids can be put into one of two categories: (1) hydrolysis of acid derivatives and (2) oxidation of various compounds.

Hydrolysis of acid derivatives. All acid derivatives can be hydrolyzed (cleaved by water) to yield carboxylic acids; the conditions required range from mild to severe, depending on the compound involved.

Oxidation. The oxidation of primary alcohols is a common method for the synthesis of carboxylic acids. This requires a strong oxidizing agent, the most common being chromic acid (H₂CrO₄), potassium permanganate (KM_nO₄), and nitric acid (HNO₃).

Other synthetic methods. Grignard reagents react with carbon dioxide (either in the gaseous form, which is bubbled through the solution, or as the solid dry ice) to give magnesium salts of carboxylic acids, which are converted to the acids themselves upon treatment with acid. Other methods for the synthesis of carboxylic acids include the malonic ester synthesis, the haloform reaction, and the Cannizzaro reaction.

REVISION EXERCISES

Ex.1. Answer the following questions:

1. What is carboxylic acid? Why was it so named? 2. Where do carboxylic acids occur in nature? 3. What are carboxylic acidВ derivatives? Where can they be found?В 4. What is the most important property of carboxylic acids? 5. Do carboxylic acids dissolve in water? 6. How does the butanoic acid smell? 7. How many classes of carboxylic acids are there? 8. Why is acetic acid so important? Where can it be found? 9. What are the most widely used ways of synthesis of carboxylic acids?ВВВВВВВВ

Ex.2. Match the words with their definitions:

1. metabolic products	a. to give something in order to help
2. acid	b. a smell, especially an unpleasant one
3. beeswax	c. any of the sequences of biochemical reactions, catalysed by enzymes, that occur in all living cells: concerned mainly with the exchange of energy and chemicals
4. donate	d. a number that cannot be divided exactly by two, for example 1,
5. odour	e. any chemical reaction in which the oxidation number of a participating chemical species changes.
6. odd number	f. a chemical substance that has aВВВ pH of less than 7; strong ones can burn holes in material or damage your skin
7.oxidation-reduction reaction	g. any of a class of chemical reactions in which an atom or group of atoms is added to a molecule
8. treatment	h. a substance produced byВВВ bees, used especially for making furnitureВВВВВВВ polishВВВВВВВВ and candles
9. addition reaction	i. technologies that use chemicals to process materials or wastes.

Ex.3. Say whether the following statements are true or false:

1. Proteins are made up of aldehydes and ketones, which also contain carboxyl groups. 2. Acyl halides, acid anhydrides, esters, and amides are the most important carboxylic acid derivatives. 3. Aspirin is prepared from carboxylic acid derivative. 4. Names of many simple carboxylic acids reflect the names of the sources they were originally isolated. 5. When common names are used, substituents on the hydrocarbon chain are designated by Greek numbers rather than by letters. 6. Acids with fewer than about five carbons are hardly dissolved in water. 7. Boiling a carboxylic acid requires more heat than boiling the corresponding alcohol. 8. People consume some carboxylic acids every morning.

Ex.4. Insert the necessary word:

1. At present, 30 elements are definitely known … to human life. 2. The most abundant elements are hydrogen, carbon, nitrogen, and oxygen; sodium, magnesium, potassium, calcium, phosphorus, sulfur, and chlorine are also present …. 3. Although present only in trace amounts, the first-row transition metals are essential for the action of … (biological catalysts). In time, other elements will probably be found to be essential. 4. Life is organized around the functions of …, the smallest unit in living things that exhibits the properties normally associated with life, such as reproduction, …, mutation, and sensitivity to external stimuli. 5. As the fundamental building blocks of all living systems, aggregates of cells form …, which in turn are assembled into the organs that make up complex living systems. 6. Therefore, to understand how life is …, we must learn how cells operate on the molecular level. This is the main thrust of biochemistry. (maintained and reproduced, in relatively large amounts, to be essential, the cell, metabolism, tissues, many enzymes.)

 

UNIT XIV ESTERS

ВВ Esters are chemical compounds derived from an acid (organic or inorganic) in which at least one -OH (hydroxyl) group is replaced by an -O-alkyl (alkoxy) group. Esters derived from carboxylic acids are the most common. The general formula is RCOOR' (R and R' are any organic combining groups).В The term ester was introduced in the first half of the 19th century by German chemist Leopold Gmelin.

ВВ Nomenclature. Ester names are derived from the parent alcohol and the parent acid, where the latter may be organic or inorganic. Esters derived from the simplest carboxylic acids are commonly named according to the more traditional, so-called "trivial names" e.g. formate, acetate, propionate, and butyrate, as opposed to the IUPAC nomenclature methanoate, ethanoate, propanoate and butanoate. Esters derived from more complex carboxylic acids are, on the other hand, more frequently named using the systematic IUPAC name, based on the name for the acid followed by the suffix -oate, for example, hexyl octanoate, Cyclic esters are called lactones, regardless of whether they are derived from an organic or an inorganic acid. One example of a (organic) lactone is gamma -valerolactone.

ВВ The names can also be derived from an inorganic acid and an alcohol. Thus, the nomenclature extends to inorganic oxo acids, e.g. phosphoric acid, sulfuric acid, nitric acid and boric acid. For example, triphenyl phosphate is the ester derived from phosphoric acid and phenol. Organic carbonates are derived from carbonic acid; for example, ethylene carbonate is derived from carbonic acid and ethylene glycol.

ВВ Physical properties. Esters contain a carbonyl center, which gives rise to 120В°C-C-O and O-C-O angles. Unlike amides, esters are structurally flexible functional groups because rotation about the C-O-C bonds has a low barrier. Their flexibility and low polarity is manifested in their physical properties; they tend to be less rigid (lower melting point) and more volatile (lower boiling point) than the corresponding amides.

ВВ Esters are more polar than ethers but less polar than alcohols. They have dipole-dipole interactions as well as van der Waals dispersion forces. Their boiling points are not high. They participate in hydrogen bonds as hydrogen-bond acceptors, but cannot act as hydrogen-bond donors, unlike their parent alcohols. Because of their lack of hydrogen-bond-donating ability, esters do not self-associate. The ability to participate in hydrogen bonding confers some water solubility. But solubility decreases with the chain length. The reason for the solubility is that although esters can't hydrogen bond with themselves, they can hydrogen bond with water molecules.

ВВ Preparation. Esterification is the classic synthesis for ester preparation. It involves treating a carboxylic acid with an alcohol in the presence of a dehydrating agent. In the reaction the hydroxyl group (OH) of the carboxylic acid is replaced by the alkoxy group (R′O) of the alcohol. The reaction is slow in the absence of a catalyst. Sulfuric acid is a typical catalyst for this reaction. Many other acids are also used such as polymeric sulfonic acids. Since esterification is highly reversible, the yield of the ester can be improved using Le Chatelier's principle:

В· Using the alcohol in large excess (i.e., as a solvent).

В· Using a dehydrating agent: sulfuric acid not only catalyzes the reaction but sequesters water (a reaction product). Other drying agents such as molecular sieves are also effective.

В· Removal of water by physical means such as distillation.

ВВ The reverse of the esterification reaction is an example of hydrolysis. Esters may also be obtained by reaction of acid halides or acid anhydrides with alcohols or by reaction of salts of carboxylic acids with alkyl halides. One ester may be converted to another ester by reaction with an alcohol, a carboxylic acid, or a third ester in the presence of a catalyst. The hydrolysis of esters in the presence of alkalies, a reaction called saponification, is utilized in the preparation of soaps from fats and oils and is also used for the quantitative estimation of esters.

ВВ Applications. Esters are widespread in nature and are widely used in industry. They are responsible for the aroma of many fruits, including apples, durians, pears, bananas, pineapples, and strawberries.

ВВ Esters of low molecular weight are colourless, volatile liquids with pleasant odours, slightly soluble in water. They are commonly used as fragrances and found in essential oils and pheromones. Many are responsible for the fragrance and flavour of flowers and fruits; for example, isopentyl acetate is present in bananas, methyl salicylate in wintergreen, and ethyl butyrate in pineapples. These and other volatile esters with characteristic odours are used in synthetic flavours, perfumes, and cosmetics. Certain volatile esters are used as solvents for lacquers, paints, and varnishes; for this purpose, large quantities of ethyl acetate and butyl acetate are commercially produced. Waxes secreted by animals and plants are esters formed from long-chain carboxylic acids and long-chain alcohols. Fats and oils are esters of long-chain carboxylic acids and glycerol. Glycerides, which are fatty acid esters of glycerol, are important esters in biology, being one of the main classes of lipids, and making up the bulk of animal fats and vegetable oils.

ВВ Liquid esters of low volatility serve as softening agents for resins and plastics. Esters also include many industrially important polymers. Polymethyl methacrylate is a glass substitute sold under the names Lucite and Plexiglas; polyethylene terephthalate is used as a film (Mylar) and as textile fibres sold as Terylene, Fortrel, and Dacron. Several billion kilograms of polyesters are produced industrially annually, important products being polyethylene terephthalate, acrylate esters, and cellulose acetate.

ВВ Nitrate esters (e.g., glyceryl trinitrate, or nitroglycerin) are explosive. ВВВ Phosphate esters are biologically important (nucleic acids belong to this group) forming the backbone of DNA. TheyВ are used widely in industry as solvents, plasticizers, flame retardants, gasoline and oil additives, and insecticides.

ВВ Esters of sulfuric and sulfurous acids are used in the manufacture of dyes and pharmaceuticals. Dimethyl sulfate, the best-known ester of sulfuric acid, is a dangerous poison.

REVISION EXERCISES

ВВ Ex.1. Answer the following questions:

1. When was the term ester introduced? 2. What esters are the most common?В 3. What are the names of cyclic esters derived from? 4. What influences the decrease of solubility of esters? 5. Are esters soluble in water? Why? 6. What is the classic synthesis for ester preparation? 7. What catalysts are used for ester preparation?

Ex.2. Match the words with their definitions: В

1) acid	a) a compound that contributes to the flavors and aromas in fruits and flowers;
2) nomenclature	b) a method of separating mixtures based on differences in their volatilities in a boiling liquid mixture;
3) ester	c) alkaline hydrolysis of the fatty acid esters;
4) amide	d) a chemical substance whose aqueous solutions are characterized by sour taste; the ability to turn blue litmus red and the ability to react with bases;
5) polarity	e) a system of names or terms or the rules for forming these terms;
6) distillation	f) a separation of electric charge leading to a molecule or its chemical groups having an electric dipole or multiple moment;
7) saponification	g) functional group containing a carbonyl group linked to a nirtogen atom.

Ex.3. Say whetherВ the following statements are true or false:

1. Esters are chemical compounds derived from an organic or inorganic acid. 2. Esters derived from the simplest carboxylic acids are commonly named using the systematic IUPAC names. 3. Esters contain a chiral centre which gives rise to 120^o C-C-O and O-C-O angles. 4. Esters are more polar than ethers but are less polar than alcohols. 5. Esters participate in hydrogen bonds as hydrogen-bond acceptors. 6. Esters can't hydrogen bond with water molecules. 7. Esters are responsible for the aroma of many fruits.

Ex.4. Insert the necessary word:

1. Ester names are derived from the parent... and the parent acid. 2. Flexibility and low... of esters is manifested in their physical properties. 3. Esters cannot act as hydrogen-bond... unlike their parent alcohols. 4.... is the classic synthesis for ester preparation. 5. Sulfuric acid is a typical... for the reaction of esterification. 6. Esters are commonly used as... and found in essential oils and pheromones. 7. Fats and oils are... of long-chain carboxylic acids and glycerol. (esterification, polarity, esters, catalysts, alcohol, fragrances, donors) ВВ

UNIT XV CARBOHYDRATES

Carbohydrates (also called saccharides) are molecular compounds made from just three elements: carbon, hydrogen and oxygen. Monosaccharides (e.g. glucose) and disaccharides (e.g. sucrose) are relatively small molecules. They are often called sugars. Other carbohydrate molecules are very large (polysaccharides such as starch and cellulose).

Carbohydrates are:

• a source of energy for the body e.g. glucose and a store of energy, e.g. starch in plants
• building blocks for polysaccharides (giant carbohydrates), e.g. cellulose in plants and glycogen in the human body
• components of other molecules e.g. DNA, RNA, glycolipids, glycoproteins, etc.

Monosaccharides are the simplest carbohydrates and are often called single sugars. They are the building blocks from which all bigger carbohydrates are made.

Monosaccharides have the general molecular formula (CH₂O) _n, where n can be 3, 5 or 6. They can be classified according to the number of carbon atoms in a molecule:

n = 3	trioses, e.g. glyceraldehyde
n = 5	pentoses, e.g. ribose and deoxyribose ('pent' indicates 5)
n = 6	hexoses, e.g. fructose, glucose and galactose ('hex' indicates 6)

There is more than one molecule with the molecular formula C₅H₁₀O₅ and more than one with the molecular formula C₆H₁₂O₆. Molecules that have the same molecular formula but different structural formulae are called structural isomers.

Glyceraldehyde's molecular formula is C₃H₆O₃. Its structural formula shows it contains an aldehyde group (-CHO) and two hydroxyl groups (-OH). The presence of an aldehyde group means that glyceraldehyde can also be classified as an aldose. It is a reducing sugar and gives a positive test with Benedict's reagent.

Pentoses and hexoses can exist in two forms: cyclic and non-cyclic. In the non-cyclic form their structural formulae show they contain either an aldehyde group or a ketone group.

Monosaccharides containing the aldehyde group are classified as aldoses, and those with a ketone group are classified as ketoses. Aldoses are reducing sugars; ketoses are non-reducing sugars. This is important in understanding the reaction of sugars with Benedict's reagent.

Glucose is the most important carbohydrate fuel in human cells. Its concentration in the blood is about 1 gdm^-3. The small size and solubility in water of glucose molecules allows them to pass through the cell membrane into the cell.

There are two forms of the cyclic glucose molecule: О±-glucose and ОІ-glucose. Two glucose molecules react to form the dissacharide maltose. Starch and cellulose are polysaccharides made up of glucose units.

Galactose molecules look very similar to glucose molecules. They can also exist in О± and ОІ forms.

Fructose, glucose and galactose are all hexoses. However, whereas glucose and galactose are aldoses (reducing sugars), fructose is a ketose (a non-reducing sugar). It also has a five-atom ring rather than a six-atom ring. Fructose reacts with glucose to make the dissacharide sucrose.

Ribose and deoxyribose are pentoses. The ribose unit forms part of a nucleotide of RNA. The deoxyribose unit forms part of the nucleotide of DNA.

Monosaccharides are rare in nature. Most sugars found in nature are disaccharides. These form when two monosaccharides react.

A condensation reaction takes place releasing water. This process requires energy. A glycosidic bond forms and holds the two monosaccharide units together.

The three most important disaccharides are sucrose, lactose and maltose. They are formed from the a forms of the appropriate monosaccharides. Sucrose is a non-reducing sugar. Lactose and maltose are reducing sugars.

Disaccharides are soluble in water, but they are too big to pass through the cell membrane by diffusion. They are broken down in the small intestine during digestion to give the smaller monosaccharides that pass into the blood and through cell membranes into cells.

Monosaccharides are used very quickly by cells. However, a cell may not need all the energy immediately and it may need to store it. Monosaccharides are converted into disaccharides in the cell by condensation reactions. Further condensation reactions result in the formation of polysaccharides. These are giant molecules which, importantly, are too big to escape from the cell. These are broken down by hydrolysis into monosaccharides when energy is needed by the cell.

Monosaccharides can undergo a series of condensation reactions, adding one unit after another to the chain until very large molecules (polysaccharides) are formed. This is called condensation polymerisation, and the building blocks are called monomers. The properties of a polysaccharide molecule depend on:

• its length (though they are usually very long)
• the extent of any branching (addition of units to the side of the chain rather than one of its ends)
• any folding which results in a more compact molecule
• whether the chain is 'straight' or 'coiled'

Starch is often produced in plants as a way of storing energy. It exists in two forms: amylose andamylopectin. Both are made from О±-glucose. Amylose is an unbranched polymer of О±-glucose. The molecules coil into a helical structure. It forms a colloidal suspension in hot water. Amylopectin is a branched polymer of О±-glucose. It is completely insoluble in water.

Glycogen is amylopectin with very short distances between the branching side-chains. Starch from plants is hydrolysed in the body to produce glucose. Glucose passes into the cell and is used in metabolism. Inside the cell, glucose can be polymerised to make glycogen which acts as a carbohydrate energy store.

Cellulose is a third polymer made from glucose. But this time it's made from ОІ-glucose molecules and the polymer molecules are 'straight'.

Cellulose serves a very different purpose in nature to starch and glycogen. It makes up the cell walls in plant cells. These are much tougher than cell membranes. This toughness is due to the arrangement of glucose units in the polymer chain and the hydrogen-bonding between neighbouring chains.

Cellulose is not hydrolysed easily and, therefore, cannot be digested so it is not a source of energy for humans. The stomachs of Herbivores contain a specific enzyme called cellulase which enables them to digest cellulose.

Carbohydrate benefits

The right kind of carbohydrates can be incredibly good for you. Not only are they necessary for your health, but they carry a variety of added benefits.

Mental health

Carbohydrates may be important to mental health. A study published in 2009 in the journal JAMA Internal Medicine found that people on a high-fat, low- carbohydrates diet for a year had more anxiety, depression and anger than people on a low-fat, high- carbohydrates diet. Scientists suspect that carbohydrates help with the production of serotonin in the brain.

Carbohydrates may help memory, too. A 2008 study at Tufts University had overweight women cut carbohydrates entirely from their diets for one week. Then, they tested the women's cognitive skills, visual attention and spatial memory. The women on no- carbohydrates diets did worse than overweight women on low-calorie diets that contained a healthy amount of carbohydrates.

Weight loss

Though carbohydrates are often blamed for weight gain, the right kind of carbohydrates can actually help you lose and maintain a healthy weight. This happens because many good carbohydrates, especially whole grains and vegetables with skin, contain fiber. It is difficult to get sufficient fiber on a low- carbohydrates diet. Dietary fiber helps you to feel full, and generally comes in relatively low-calorie foods.

A study published in the Journal of Nutrition in 2009 followed middle-age women for 20 months and found that participants who ate more fiber lost weight, while those who decreased their fiber intake gained weight. Another recent study linked fat loss with low-fat diets, not low- carbohydrates ones.

Good source of nutrients

Whole, unprocessed fruits and vegetables are well known for their nutrient content. Some are even considered superfoods because of it — and all of these leafy greens, bright sweet potatoes, juicy berries, tangycitruses and crunchy apples contain carbohydrates.

One important, plentiful source of carbohydrates is whole grains. A large study published in 2010 in the Journal of the American Dietetic Association found that those eating the most whole grains had significantly higher amounts of fiber, energy and polyunsaturated fats, as well as all micronutrients (except vitamin B12 and sodium). An additional study, published in 2014 in the journal Critical Reviews in Food Science and Nutrition, found that whole grains contain antioxidants, which were previously thought to exist almost exclusively in fruits and vegetables.

REVISION EXERCISES

Ex.1. Answer the following questions:

1. What are carbohydrates? 2. What functions do carbohydrates perform? 3. What are structural isomers? 4. What is the difference between aldoses and ketoses? 5. What do the properties of a polysaccharide molecule depend on? 6. What are the carbohydrate benefits for mental health? 7. What did a large study published in 2010 in the Journal of the American Dietetic Association find? 8. What did an additional study, published in 2014 in the journal Critical Reviews in Food Science and Nutrition discover?

Ex.2. Match the words with their definitions:

1. sugar	a. the process of breathing;
2.Benedict's reagent	b. a substance that carriesВВВВВ geneticВВВВВВВВ information in the cells of the body;
3. fuel	c. to divide into parts or categories or to separate (as a chemical compound) into simpler substances;
4.respiration.	d. a sweet white or brown substance that is obtained from plants and used to make food and drinks sweet;
5. RNA	e. to be the cause of (a situation, action, or state of mind);
6. DNA	f. is a polymeric molecule implicated in various biological roles in coding, decoding, regulation, and expression of genes;
7. break down	g. a chemical commonly used to detect presence of reducing sugars, however other reducing substances also give a positive reaction;
8. result in	h. a substance such as coal, gas, or oil that can be burned to produce heat or energy.

Ex.3. Say whether the following statements are true or false:

1. Monosaccharides and disaccharides are often called sugars. 2. Monosaccharides have the general molecular formula (CH2O)n, where n can be 2, 4 or 8. 3. Monosaccharides containing the aldehyde group are classified as ketoses. 4. The most important carbohydrate fuel in human cells releases energy when the molecules are metabolized. 5. Fructose reacts with glucose to make the dissacharide lactose.6. Most sugars found in nature are monosaccharides. 7. The women's cognitive skills, visual attention and spatial memory are better if they consume a healthy amount of carbohydrates every day.

Ex.4. Insert the necessary word:

1. Starch is manufactured in the green leaves of plants from excess glucose produced during …. 2. It serves the plant as …. 3. Starch is stored in chloroplasts in the form of granules and in such organs as … of the tapioca plant; … of the potato; … pith of sago; and … of corn, wheat, and rice. 4. When required, starch is broken down, in the presence of certain … and water, into its constituent monomer glucose units, which diffuse from the cell … the plant tissues. 5. In humans and other animals, starch is broken down into its … sugar molecules, which then supply energy to the …. (the roots, tissues, photosynthesis, the tuber, constituent, to nourish, a reserve food supply, the seeds, enzymes)

UNIT XVI Fats

ВFats occur naturally in food and play a significant role in human nutrition. Fats are used to store energy in the body, insulate body tissues, cushion internal organs, and transport fat-soluble vitaminsin the blood. Fats also play in an important role in food preparation: They enhance food flavor and food texture, make baked products tender, and conduct heat during cooking.

Fats are the most prevalent class of compounds (in living systems) referred to as lipids. Lipids are cellular compounds that are insoluble in water. Fats are soft, low-melting solids, with a density less than that of water. They have a greasy feel and are slippery. Because fats are insoluble in water and less dense than water, after meat that has a lot of fat in it has been cooked, upon cooling a layer of fat often appears on top of the juices. Fats and closely related oils are mixtures of compounds consisting of fatty acids combined with glycerol (commonly known as glycerin) via esterlinkages. Fatty acids are long, straight chain carboxylic acids. A fat (or oil) is formed when three fatty acid molecules react with a glycerol molecule to yield a triglyceride (and three water molecules). Fats in the body are transported and stored as triglycerides.

Fat molecules are characterized as monoglycerides, diglycerides, or triglycerides, depending on whether there are one, two, or three fatty acid chains present in the molecules. Fatty acids in nature generally have an even number of carbon atoms because they are synthesized in cells via successive additions of two-carbon acetate groups in a stepwise cyclic reaction.

Fats and Oils

Dietary fats and oils are both triglycerides. Fats are generally solids and oils are generally liquids at ordinary room temperatures. The characteristics of fats and oils are related to the properties of the fatty acids that they contain. The larger the number of carbon atoms, the higher the melting point; the larger the number of double bonds, the lower the melting point. Oils contain a higher percentage of unsaturated fatty acids than fats. Fats from animal sources tend to be solids and fats from vegetable sources tend to be liquids. Thus fats are often referred to as "animal fats" and "vegetable oils."

When fats or oils are exposed to air, they react with the oxygen or water vapor to form short-chain carboxylic acids. The short-chain acids are volatile and have unpleasant smells and tastes. For example, the strong smell and sour taste of vinegar are due to acetic acid, a two-carbon carboxylic acid. The oxidationprocess is called rancidification and can make foods unpalatable. The characteristic smell of rancid butter is due to the presence of butyric acid (a four-carbon acid). (Rancidity can also be the result of the hydrolysis of fats or oils.)