Chemistry UNIT 4 AOS 1

Diversity of carbon compounds

• All 4 of its valence electrons are available for bonding 

• Forms chains and rings via single, double or triple bonds with itself 

• Can bond with other non-metals (E.g. O, N, S, P, Cl)

• Forms strong bonds that require lots of energy to break

Bond strength = Bond energy (energy in kJ needed to break 1 mole of a bond in the gaseous state) 

Higher = Stronger + More stable bond

Saturated hydrocarbons

Contains only single covalent bonds between carbon atoms

Unsaturated hydrocarbons

Contains at least one double or triple bond between carbon atoms

Isomers

Molecules with the same number and type of atoms, but with different arrangements of these atoms

• Chain isomers = Have different chain lengths due to branching

• Positional isomers = Form when a branch or functional group is moved

• Stereoisomers = Form when groups around a central atom are arranged differently in 3D space

How many isomers?

• Consider both chain length, position and functional groups

Different representations

Representation	Example (Propene)
Molecular formula	C3H6
Structural formula
Semi-structural formula (condensed formula)	CH3CHCH2
Skeletal formula

Homologous series

Family of organic molecules that have similar structures and properties

Alkanes

• Chains are named with a prefix for number of Cs and the suffix -ane

• Side branches are named using a prefix and the suffix –yl

• Use the prefixes di, tri, tetra etc. for multiple branches of one type

• Branch position is found by numbering main chain Cs

• Give branches the lowest numbers possible

• List branches in alphabetical order Commas between numbers, dashes between numbers and words

• No spaces!

Prefixes

Carbon Atoms	Prefix
1	Meth-
2	Eth-
3	Prop-
4	But-
5	Pent-
6	Hex-
7	Hept-
8	Oct-
9	Non-
10	Dec-

Cycloalkanes

Rings with only single bonds between carbons

Alkenes

Hydrocarbons with one or more C to C double bonds

• Names end in –ene

• 2 Hs are lost for each C to C double bond

• Double bond position must be given using the lowest number possible

• Insert the double bond position before –ene 

• Side branches are named as for alkanes

Degree of unsaturation

The number of double bonds or ring structures in the molecule

• Every double bond or ring leads to 2 fewer Hs than in a saturated molecule of the same length

Benzene

• Unsaturated 6 C ring 

• Fourth electron in the valence shell of each C becomes delocalized and is shared by all Cs

• You can think of each bond between Cs as a one-and-a-half bond

Haloalkanes

Alkanes with one or more H atoms replaced by a halogen atom, such as Cl, I, Br or F

• The C–halogen bond is a single polar covalent bond

• Asymmetrical haloalkanes are polar molecules

• Prefixes (fluoro, chloro, bromo, iodo) are added to the alkane name

• The location of all halogen atoms must be given

• Number the chain to give halogen the lowest possible number

• For multiple atoms of the same halogen use the prefixes di, tri, tetra

Alcohols

Contain the hydroxyl (–OH) functional group

• Classified based on their structure

Primary (1°) alcohols: C bonded to the –OH group is bonded to one alkyl chain (carbon containing side group, or ‘R’ group) 

Secondary (2°) alcohols: C bonded to the –OH is bonded to two alkyl chains

Tertiary (3°) alcohols: C bonded to the –OH group is bonded to three alkyl chains

• The polar O–H bond allows for H-bonding between molecules

• The C–O bond is also polar and makes alcohols quite reactive 

• Names have the suffix –ol, or in some cases the prefix –hydroxy

• Insert a number before –ol to indicate the position of –OH groups 

• Use the prefix di, tri, tetra, also before –ol, for multiple –OH groups

• Number a chain to give –OH groups the lowest number possible

Amines

Contain the amino functional group –NH₂

• Classified based on the number of alkyl chains attached to the N atom

Primary amines: N is bonded to one alkyl chain and two Hs

• Names end in –amine

• Indicate the position of the amino group with the lowest number possible

Carbonyl group

Consists of a C double bonded to an O. The group is polar and the angle between bonds is 120°

• Carbonyl-containing compounds include amides, aldehydes, ketones, carboxylic acids and esters

Aldehydes

Consist of a carbonyl group located at one end of an alkyl chain

• The C of the carbonyl group is also bonded to one H

• The group is written –CHO in semi-structural formulas  

• The carbonyl C is given the number 1 and doesn’t need its position labelled

• Names end in –al and sometimes have the prefix oxo–

Ketones

Contain a carbonyl group where the C is bonded to two other alkyl chains

• The carbonyl group is never at the end of a chain

• The group is written –CO– in semi-structural formulas

• Names end in –one or may have the prefix oxo –

• Carbonyl group position is always indicated. The simplest ketone is named propan-2-one, even though there is no other position possible

Propan-2-one is also called acetone

Carboxylic acids

Contain the carboxyl functional group (–COOH), which consists of the C of a carbonyl group bonded to a hydroxyl group

• The C in a carboxyl group is always on the end of a chain

• The carboxyl group C is always C no 1, so its position isn’t indicated

• Names end in –oic acid

Amides

Contain the amide functional group (–CONH₂), which consists of an amine group bonded to the C of a carbonyl group.

• Amides are usually derived from carboxylic acids

• In primary amides, N is bonded to one alkyl chain 

• The amide group must be on one end of a carbon chain

Esters

Formed from a condensation reaction between a carboxylic acid and an alcohol

In semi-structural formulas the ester functional group is written as –COO – or –OCO –

• Esters have two-part names. 

• The first part includes the prefix for the C chain single bonded to O. This is derived from the alcohol reactant and is given the suffix –yl. 

• The second part includes the prefix for C chain starting with the carbonyl group. It comes from the carboxylic acid and gets the suffix –oate

Systematic naming conventions

• The longest unbranched carbon chain is the ‘parent molecule’

• The parent molecule must include most functional groups

• Functional groups are named with a suffix added to the parent name

• Alkyl side chains and halogen atoms are considered branches off the main chain. They are named with prefixes added to the parent name

• Positions of branches and some functional groups are indicated by numbering the parent chain Cs

• If multiple prefixes are added to the parent name, they are listed alphabetically

• No spaces, except in ester and carboxylic acid names

• Numbers separated by commas, numbers and words separated by dashes

Examples to practice

Physical properties

Organic compounds vary in their melting points, boiling points and viscosities and this affects their uses

Consider:

• Molecule size 

• Molecule shape 

• The type of bonding between molecules

Properties of alkanes

Hydrocarbons, so they are non-polar

• Dispersion forces are found between molecules

As chain length increases, melting and boiling points increase

As C chain length increases:  

• There are more points of contact between molecules 

• The strength of temporary dipoles increases 

Thus, dispersion forces are stronger between heavier molecules. More heat energy must be added to break them so melting and boiling points increase.

Linear molecules pack closely with more surface area for contact compared with branched molecules

• Dispersion forces are therefore stronger in linear molecules compared with branched molecules

Viscosity

Resistance to pouring

• Depends on forces of attraction between molecules

• As chain length increases, so does viscosity

Property of alkenes

Hydrocarbons with dispersion forces between molecules

• MP and BP is very similar to that of alkanes of a similar length, and trends are the same as in alkanes

Alkenes have fewer electrons than alkanes of the same length. The double bond also changes molecular shape (e.g. cis isomers) and this may prevent molecules from packing closely.

Properties of haloalkanes

Follow similar trends to alkanes

• Asymmetrical haloalkanes are polar molecules, so dipole-dipole interactions form between them

• These are stronger than dispersion forces, so MP and BP are higher

Properties of alcohols

In alcohols, there are hydrogen bonds between molecules

• These are stronger than both dispersion forces and dipole-dipole forces

• MP and BP of alcohols is higher than that of alkanes of the same mass

• Viscosity of alcohols is higher than that of alkanes of the same mass

Primary alcohols have higher BPs than secondary and tertiary alcohols of similar size. This is due to the placement of the hydroxyl group.

In secondary and tertiary alcohols, the –OH group is “crowded” by other atoms, restricting the formation of H bonds with other molecules.

Properties of amines and amides

• Hydrogen bonds also form between amines and amides

• Have higher MPs, BPs and viscosity than hydrocarbons of a similar size

Properties of carboxylic acids

Have higher boiling points than even alcohols of a similar size. This is because they form hydrogen bonded dimers

Each dimer acts like a single molecule with double the mass of the carboxylic acid

• Results in relatively strong dispersion forces between dimers

• Carboxylic acids have higher BPs than alcohols due to stronger dispersion forces between molecules as a result of dimerisation

Properties of ketones, aldehydes and esters

Contain the polar carbonyl group, but they do not contain O–H bonds

• Thus dipole-dipole bonding rather than H bonding forms between molecules

Their BPs are higher than alkanes but lower than alcohols of the same size

Combustion

In excess oxygen, alkanes, alkenes and alcohols undergo combustion to produce CO₂ and H₂O

• Be able to write complete + incomplete combustion

Substitution and alkanes

When an atom or functional group becomes replaced by another

• Alkanes undergo substitution reactions with halogens (e.g. F₂, Cl₂, Br₂) in the presence of UV light to produce haloalkanes and hydrogen halides

• Any H atoms in the reacting alkane can be substituted

• They are replaced one at a time, and molecules can react multiple times   

• A reaction mixture therefore contains a mix of products

• Different isomers can result depending on which H atoms are replaced in an alkane

Substitution of haloalkanes

Haloalkanes are much more reactive than alkanes

• The C bonded to the halogen is δ+ and can thus be ‘attacked’ by negatively charged particles such as an OH– ion or the δ– N of NH₂

• A negative particle that can share a pair of electrons with a δ+ carbon is called a nucleophile

• The OH– ion is a strong nucleophile

Ammonia (NH₃) can also be a nucleophile

NH₃ reacts with haloalkanes to produce an amine and a hydrogen halide

• The halogen atom is always replaced in its original position

• Water is a weak nucleophile

• It can react with haloalkanes but requires a catalyst or heat for the reaction to proceed

Addition reactions of alkenes

Alkenes are more reactive than alkanes due to having a double bond. Addition reactions are possible.

In these reactions:

1. The inorganic reactant breaks into two atoms or groups  

2. Each group forms a bond to one C involved in the alkene double bond

3. The double bond becomes a single bond

Addition with hydrogen (H₂) 

• Alkenes react with hydrogen gas in the presence of a solid Ni or Pt catalyst to form alkanes

• This is called a hydrogenation reaction

• An unsaturated molecule is converted to a saturated molecule

Addition with halogens  

• Halogens, such as Br₂, Cl₂ and I₂, can be added across a double bond 

• These reactions occur at room temperature without a catalyst

The bromine test 

• Bromine can be added to organic compounds to test for the presence of a double bond

• Br₂ is reddish brown

• Once added across a double bond, the haloalkane formed is colourless

Addition with hydrogen halides  

• Hydrogen halides, such as HCl, add across a double bond like halogens

• If the alkene is symmetrical, such as but-2-ene, there is one product

• If the reacting alkene is asymmetrical, two different isomers are possible in the products

•  But-1-ene, for instance, reacts with HCl to form both 2-chlorobutane and 1-chlorobutane

Addition with water 

• Water can add across a double bond to produce an alcohol

• This requires a catalyst such as solid phosphoric acid and high temperatures (300°C)

• As with hydrogen halides, multiple isomers can form when water reacts with asymmetrical alkenes

Oxidation of alcohols

• When organic molecules are oxidised, C-H bonds are broken and replaced with C-O bonds

• C atoms with more C-O bonds are said to be more oxidised

Alcohols can be oxidised to ketones, aldehydes and carboxylic acids by strong oxidising agents such as acidified permanganate (H⁺/MnO₄^–) or dichromate (H⁺/Cr₂O₄^2-) ions.

• Unbalanced equations are often used to represent these reactions

• Structural formulas are used for organic species, inorganic reactants go above the arrow

Oxidation of primary alcohols

• When primary alcohols are reacted with oxidants, they are first converted to aldehydes and if allowed to react more, become carboxylic acids

• Sometimes the intermediate stage is skipped, and equations show primary alcohols being converted directly to carboxylic acids

Oxidation of secondary alcohols

• When secondary alcohols are oxidised, ketones are formed

Oxidation of tertiary alcohols

• Tertiary alcohols do not undergo oxidation when combined with strong oxidants 

• There are no C-H bonds to contribute electron pairs for the  conversion of the –OH to a =O group

How oxidation of alcohols can be shown (Prac application)

• The oxidants usually reacted with alcohols have strong colours that change with their oxidation state

• This means they can be combined with alcohols and their colour can indicate if a reaction has occurred

Formation of esters

Esters are formed when an alcohol reacts with a carboxylic acid in the presence of a concentrated sulfuric acid catalyst and heat

The H in the alcohol hydroxyl group combines with the OH from the carboxylic acid to form H₂O as a by-product

• This is a type of condensation reaction, known as esterification

Hydrolysis of esters

• Esterification is reversible

• H₂O can react with an ester bond to break it into a carboxylic acid and alcohol

• This hydrolysis reaction occurs in the presence of heat and a dilute acid or alkaline catalyst

• Dilute acid catalyst: Products are an alcohol and a carboxylic acid

• Dilute alkali catalyst: Products are a salt of the carboxylic acid and an alcohol. The salt can be converted to a carboxylic acid by adding a dilute acid

Triglycerides

A fat molecule consisting of three long hydrocarbon chains attached to a three-carbon backbone through ester bonds

Transesterification

• Triglycerides undergo transesterification when they react with alcohols

• The alcohol R group swaps positions with the R group attached to the –O– of the ester

Biodiesel

When triglycerides in plant or animal fats are reacted with methanol in the presence of a KOH catalyst, the fatty acid esters (methyl esters) formed can be used as a fuel - biodiesel

Reaction Pathway

A series of steps to convert starting materials (e.g. alkanes, alkenes) into desired products

E.g. ethanol from ethene:

• Carboxylic acids can be synthesised from an alkanes in several steps

• There are also other methods

• A pathway for making ethyl propanoate from ethene and propane

Sample Question + Answer

VCAA 2011 Question:

Banana oil, 3-methylbutylethanoate, is a fragrant liquid that gives bananas their characteristic odour. A chemist working for ‘Go Bananas’ has proposed the following pathway for the synthesis of banana oil.

Answer:

Considerations (Things to ask when designing a reaction pathway)

• What catalysts and special conditions are required?

• Will a mix of isomers be produced?

• What by-products will there be?

• How can unwanted isomers and by-products be separated out?

• How pure will the final product be after separation?

• What are the equilibrium constants for the various steps?

• What is the yield and atom economy of the desired product?

Percentage Yield

Measures its efficiency through the actual mass of product obtained as a proportion of the maximum theoretical mass based on mole ratios in the balanced equation

• Make sure you use the limiting reagent to find theoretical yield

• A high percentage yield is important to avoid wasting resources

Overall Percentage Yield

The percentage yield of each step multiplied together

• A low yield in one step can significantly affect the overall yield

Why might the actual yield be lower than the theoretical yield?

• Slow reaction rate 

• Reaction reaches equilibrium rather than going to completion 

• Loss of reactants or products due to transfers between containers 

• Competing or side reactions forming unwanted products

Atom Economy

The proportion of atoms in reactants that become useful products

• Provides a measure of the waste from a reaction

• Ideally reactions used in industry will have an atom economy of 100%

• Addition reactions, for example, meet this standard

• Wasteful reactions have a low atom economy

Atom economy is one the American Chemical Society’s principles of green chemistry. It states that:  

• Chemical processes should aim to incorporate all reactants into the final product 

• It is better to prevent waste than to treat it or clean it up

Green Chemistry Principles

• Use of renewable feedstocks 

• Catalysis 

• Designing safer chemicals

Renewable feedstocks

Fossil fuel sources will eventually run out. As demand for products rises, a switch to renewable sources is essential

• Some bio-based polymers (from renewable plant sources) are also biodegradable, which means they do not persist in the environment as wastes

Catalysts

The benefits for catalysts in chemical production:  

• They allow reactions to be carried out at lower temperatures, thus reducing heating costs which saves energy resources

• They increase reaction rates, meaning more product in a shorter time

• They are not consumed in the reaction so can be used continuously

Safer chemicals

Low impact on both humans and the environment

E.g. Perfluoroalkyls (PFAS) are a group of substances used in firefighting foam. 

• They have been found to persist in the environment and may be toxic to humans and other animals.  

• Some states have banned PFAS in firefighting Replacements are being investigated

Condensation polymerisation

Condensation reactions occur when two functional groups react and a small molecule, such as water, is formed as a by-product. 

• In condensation polymerisation, many monomers combine through this reaction

Homopolymers contain of one type of monomer

Copolymers contain two or more different monomers

Proteins

Polymers of amino acids

General structure:

• The amino acids in proteins are called 2-amino acids or α-amino acids because the main functional groups are attached to the number 2 carbon

• In solution, amino acids often have a charge. In a neutral solution, both positive and negative charges are present with an overall zero charge

• The side chain or R group differs in the 20 amino acids. This group may have special properties

Peptide bond

Forms when two amino acids combine through a condensation reaction

• Water is a byproduct

Types of peptides

Dipeptide

Two amino acids bonded together

Tripeptide

Three amino acids bonded together

Polypeptide

Many amino acids bonded together

Protein

50 or more amino acids bonded together

Polypeptide chains

• Can be represented using the three letter abbreviations for amino acids

• Every chain has a free amino group on one end, and a free carboxyl group on the other

Carbohydrates

Biomolecules with the general formula:

Types of carbohydrates

Monosaccharides

• The simplest carbohydrates

• Generally white, crystalline, sweet and soluble in water

Disaccharides

Two monosaccharides bonded together through a condensation reaction

Polysaccharides

• Long polymers of monosaccharides

• Often insoluble in water and tasteless

Starch

• Produced in plants through the action of enzymes, used for energy storage

• There are two forms, amylose (linear) and amylopectin (branched)

• Chains of amylose can coil tightly

• Amylopectin may be more soluble than amylose as –OH groups are more exposed due to its branched structure

Glycogen

• Glycogen is used in animals for energy storage

• It is a branched polymer of glucose. It is more branched than amylopectin

Lipids (Fats/Oils)

• Non-polar food molecules used for energy storage

• A major type of lipid is the triglyceride molecule (Triglycerides are made from glycerol and fatty acids)

How triglycerides are made

• Produced through condensation reactions

• Three fatty acids react with each glycerol, producing three water molecules and a triglyceride containing three ester groups

Types of triglycerides (Structure of their fatty acid chains):

Saturated fatty acids

Contain only single carbon-carbon bonds

C_nH_2n+1COOH

Monounsaturated fatty acids

Have one carbon-carbon double bond

C_nH_2n-1COOH

Polyunsaturated fatty acids

Have multiple carbon-carbon double bonds

Triglycerides (More info)

• Triglycerides are nonpolar, insoluble in water and most have two to three different fatty acid chains

• Chains differ in length and degree of saturation

Fats vs Oils

• Fats contain triglycerides with more saturated fatty acids while oils contain more unsaturated fatty acids

Hydrolysis (More info - Some repetition)

• When food is digested, the human body breaks down large food molecules into smaller molecules

• Large biomolecules are split through reactions with water molecules

• You can think of hydrolysis as the opposite of condensation

• In general, hydrolytic reactions are exothermic while condensation reactions are endothermic

Proteins undergoing hydrolysis

• When proteins undergo hydrolysis, a water molecule is added to each peptide bond (or amide link)

• The C–N bond in the peptide link breaks 

• The –OH of the water adds to the free C=O of one amino acid forming a COOH group

• The –H of the water adds to the free –NH of the other amino acid forming a NH₂ group

• In the body, proteins are broken down gradually by enzymes. If carried out in the lab without enzymes, this process requires harsh conditions (100°C for 24h, 6M HCl)

Carbohydrates undergoing hydrolysis

Water is added to glyosidic links

• One C–O bond in the link is broken and a –H from water is added to the O while the –OH from water is added to the neighbouring glucose

• The enzyme amylase catalyses the hydrolysis of starch into the disaccharide maltose

• Maltase then hydrolyses maltose to make glucose monosaccharides

• Glucose dissolves easily in the blood

Lipids (Fats and Oils) undergoing hydrolysis

• Since fats and oils are insoluble in water, they form globules in the intestines and hydrolysis only occurs at the surface

• Bile breaks globules into small droplets to increase the surface area available for hydrolysis

• Lipase catalyses hydrolysis by adding three water molecules to the ester bonds in a triglyceride

• The –OH of each water is added to the CO of each fatty acid, while each –H is added to each O of glycerol