Organic Chemistry

Fats and Oils

Triglycerides are an important group of esters known as fats and oils

Triglycerides are 3 fatty acids attached to a glycerol backbone

Fatty acids are a carboxylic acid with a long aliphatic chain

Fatty Acid

Fats and oils are formed from the esterification reaction between glycerol and fatty acids.

Production of Fats (Triglyceride)

Types of fats and oils

Solid fats

  • Straight chain fatty acids

  • Saturated or trans fatty acids

  • Fat crystals suspended in oil

Liquid fats (oil)

  • Contain kinked chain fatty acids

  • Mono-or polyunsaturated fatty acids

  • No fat crystals

Melting temperature

Melting temperature is directly proportional to degree of saturation

  • Higher saturation (less double bonds) will have a higher melting point

  • Lower saturation (more double bonds) will have a lower melting point

Animal Fats

Saturated fats (no double bond) are usually animal fats.

These have higher melting points and are more likely to be solid at room temperature.

Plant Fats

Unsaturated fats (double bond) are usually plant fats.

Lower dispersion forces

Lower melting points and are more likely to be liquid at room temperature

Hydrogenation reaction

Converts unsaturated liquid vegetable oils into more versatile solid products

Addition reaction where hydrogen atoms add to some of the double bonds

This forms an undesirable side reaction

  • Converts some of the cis double bonds into the trans form

  • The trans isomers have higher dispersion forces and allow their molecules to pack more efficiently than the cis form

Polymers

Polymers are very large organic molecules made up of small repeating units.

They are formed from reactions between small organic molecules called monomers.

The process of forming a polymer is called polymerisation.

Addition polymers are formed from certain alkenes (containing a C=C bond).

In these reactions, each molecules undergoes an addition reaction and forms a single bond with the next molecule.

Not a spontaneous reaction requires high temperatures and high pressures. This makes short chains of polyethene form and branch off the main polymer chain. Forming low density polymers which are loosely packed. This is more flexible.

If a catalyst is used and lower pressure than it produces a polymer with less branches. These are tightly packed and called high-density polymer.

Condensation polymers are formed from monomers containing two reactive functional groups, typically:

  • a carboxylic acid and alcohol

  • a carboxylic acid and amine

In these reactions, two monomer molecules combine, producing water.

The monomers must possess two reactive functional groups (e.g. one monomer with two alcohol groups and another monomer with two carboxylic acid functional group)

Polypeptides

Amino acids have the general formula:

For drawing polypeptides, we rewrite the formula as:

Amino acids join together in a condensation reaction

The bond formed between the two amino acids is called a peptide bond. When a large number of amino acids are joined by peptide bonds the resultant structure is known as a polypeptide.

Nylon

A type of condensation polymer

Polymer molecules that are composed of polyamides

Properties of nylon

  • It is strong and elastic

  • It is easy to launder

  • It dries quickly

  • It retains its shape

  • It is resilient and responsive to heat setting

Polyester

Polyester are long-chained polymers composed of ester groups in the main chain

Formed from a dicarboxylic acid and a diol

Most common is called polyethylene terephthalate (PET).

  • This is made from benzene-1,4-dicarboxylic acid and an alcohol, ethane-1,2-diol

In PET fibres, the molecules are mainly arranged in one direction, in film, they are in two directions and for packaging, they are in three directions

Uses

  • capacitators, graphics, film base and recording tapes

  • fibres for a very wide range of textile fibres

  • bottles

  • food packaging

  • electrical components

  • magnetic tape

  • backing for adhesive tape

  • sail cloth

Properties

  • Can be produced with varying degrees to crystallisation providing a range of rigidity absorbs very little water

  • Good gas barrier

  • Excellent moisture barrier

  • Chemically resistant to acids, oils, alcohol

  • Highly transparent and colourless

  • High mechanical strength

  • Low density

  • Impact resistant

Alpha amino acids

Amino acids contain a carboxyl (-COOH) group and an amine (NH2) group.

In alpha amino acids, these groups are attached to the same carbon atom.

Zwitterions

Protons move from the OH group in the COOH group and join the NH2 group to form a Zwitterion which looks like this.

These zwitterions react with acids to produce a cationic form and water.

These zwitterions react with bases to produce an anionic form and water.

Peptide formation

The amino acids form peptides by loosing the OH in the COOH group and attaching the amino group from the next monomer by removing a hydrogen ion.

Protein

Proteins are a diverse group of large and complex polymer molecules, made up of long chains of amino acids

Proteins act as:

  • Structural components of tissues (such as muscles)

  • Hormones (such as insulin)

  • Antibodies (part of the body’s immune system)

  • Biological catalysts (enzymes)

What is the difference between peptides and proteins?

Proteins do not have a “n” because they are not repeating polymers.

For proteins the sequence is important e.g. Ala-Cys-Asn is not the same as Ala-Asn-Cys.

Structure and function of proteins

Three levels of structure can be identified as contributing to a protein’s function:

  1. Primary structure: the sequence of amino acids

  2. Secondary structure: the regular arrangement of various sections of the protein chain

  3. Tertiary structure: the protein’s overall 3-D structure

Primary structure of proteins

The primary structure of proteins is the linear sequence of individual alpha amino residues

Amino acids join by forming amid (or peptide) bonds

This the part of the protein sequence containing the amino aids Alanine, Glycine and Serine

The amino acid sequence is always listed from the N-Terminus of the protein and the 3 letter abbreviation is used for each amino acid. In this case Ala-Gly-Ser.

Secondary structure of proteins

The secondary structure of a protein relates to the way individual amino acids in a protein chain bond to other amino acids in the same or adjacent chains.

Common secondary structures are:

  • α-Helix

  • β-pleated sheets

Both of these structures are held together by H-bonding.

α-Helix

The H-bonding will occur between the lone pair of electrons on the oxygen of the carbonyl (C=O) group and the polar hydrogen atom on an amide group (NH).

The amide group (NH) must be 4 residues (4 peptide links) further along the spiral from the carbonyl group (C=O) in order to form an α-Helix.

β-pleated sheets

In a beta-pleated sheet, the chains are folded so that they lie alongside each other

This is structure is based on Hydrogen bonding between the lone pairs of electrons from the oxygen of the carbonyl group and the polar hydrogen on the amide group

β-pleating will occur when the amino acids in the polypeptide chains to fit closely together

The secondary structure plays an important role in determining the functioning of the protein. Factors such as heat, pH and the presence of salts can affect the H-bonding and therefore the stability of the secondary structure.

Tertiary structure 

The tertiary structure of a protein is held together by interactions between the side chains - the “R” groups.

There are several ways this can happen including:

  • Dispersion forces between non-polar side chains

  • Hydrogen bonding between polar side chains

  • Ionic attraction between ionisable side chains such as COO- or NH3+

  • Dipole-Dipole attraction between polar side chains

  • Disulfide bridges (a covalent bond between Sulfur atoms on the amino acid cysteine)

Biodiesel

Biofuels like bioethanol and biodiesel are produced from biomass and are considered fossil fuel alternative

Biofuels are renewable resources while the petroleum they replace is not

Biofuels are essentially natural products and exposure to these is of limited or no consequence to human health or the natural environment

Petroleum based fuels are toxic and harmful when accidentally released into the environment

Carbon dioxide emissions from biofuels can be balance with the Earth’s carbon cycle as the carbon in these fuels simply replaces atmospheric carbon dioxide absorbed by plants as they produce the biomass originally used to make the fuel.

Biofuels such as bioethanol and biodiesel are considered fossil fuel alternatives

Ethanol production

There are two methods to produce ethanol

  • Ethanol production from ethene (acid catalysed)

  • Ethanol production from fermentation (enzyme catalysed) → bioethanol

Only the second method is consistent with green chemistry

Ethanol production from ethene

This is the acid-catalysed addition of water to ethene

Steam and ethene are passed over a bed of silica particles coated with pure phosphoric acid (the catalyst) at a temperature of 300 degrees and a pressure of 60-70 atm.

Use:

Production of other industrial chemicals, not used as fuel or beverage

Advantage:

This is a much quicker process than the production of ethanol from fermentation. Can be set up as a continuous process.

Disadvantage:

Produced from non-renewable petroleum

Ethanol production from fermentation (bioethanol)

Bioethanol is an alcohol produced by the natural fermentation of the carbohydrates (such as starch) in sugar beet/cane or wheat crops.

Bioethanol can be produced form the fermentation of plant sugars (produced in photosynthesis)

Glucose from photosynthesis

As this step is taking in carbon dioxide from the atmosphere, the combustion of bioethanol, which produces carbon dioxide could be considered carbon-neutral.