B3 - alkanes

Why do reactions not produce 100% yield of the desired products?

• Incomplete reaction

• Products lost during purification steps (e.g. distillation or recrystallisation)

• Products lost during transfer between apparatus

• Impure starting material

• Side reacting producing different products

Percentage yield =

(actual yield / theoretical yield) x100

What is theoretical yield?

The maximum possible amount of product

What is actual yield?

The amount of product obtained from the reaction

How is the theoretical yield calculated using the chemical equation?

Using the limiting reagent

What is percentage yield?

A measure of the efficiency of the process.

What does atom economy measure?

The efficiency of the chemical reaction

Atom economy =

(molecular mass of desired products / sum of molecular masses of all products) x 100

What will the atom economy of an addition reaction be + why?

100% as it only gives one product

How can atom economy be improved?

• Use the waste products

• Use alternate reaction with a higher atom economy

• Choose spectator ions with a lower A_r

What is fission?

Bond breaking

Homolytic fission

Breakage of a covalent bond, each bonding atom receiving one electron from the bonding pair

What does homolytic fission produce?

Radicals

What is a radical?

Species with an unpaired electron (uncharged)

What is heterolytic fission?

Breakage of a covalent bond, one bonding atom receiving electrons from the bonding pair.

What does heterolytic fission produce?

A cation and an anion

What is an electrophile?

A chemical species that will accept a pair of electrons to form a new dative covalent bond.

What is a electrophile attracted to?

An electron-rich centre/atom

What is a nucleophile?

A chemical species which donates a pair of electrons to form a new covalent bond

What is a nucleophile attracted to?

An electron-deficient centre/atom

What is an addition reaction?

Reaction in which two or more reactants combine to form a single product

What is a substitution reaction?

Reaction in which one atom or a group of atoms is replaced by another atom or group of atoms

What is an elimination reaction?

A reaction in which a small molecule is removed from a reactant. Product is an unsaturated product.

When will an alkane react with a halogen?

In the presence of UV light

What kind of reaction is this?

Substitution reaction

What are the three steps of the mechanism?

1. Initiation

2. Propagation

3. Termination

Initiation

• Br-Br bond breaks in the presence of UV radiation

• Homolytic fission of covalent bond

• 2 bromine radicals are formed

(any halogen may substitute bromine)

What is homolytic fission?

The covalent bond breaks with each atom gaining one electron

What is a radical?

An atom with an unpaired electron

Propagation

Always two steps

1. Alkane + halogen radical —> alkyl radical + hydrogen halide

2. Alkyl radical + halogen molecule —> haloalkane + halogen radical

Number of radicals remains constant - use halogen radical then regenerated one (chain reaction)

Termination

Radicals combine by colliding together randomly

always three equations

1. halogen radical + halogen radical —> halogen

2. alkyl radical + halogen radical —> haloalkane

3. alkyl radical + alkyl radical —> alkane

Products of radical substitution of alkanes reaction

1. hydrogen halide

2. haloalkane

3. longer chain alkane

Why is a mixture of products obtained from this reaction?

• Any hydrogen atom on the carbon chain can be substituted

• Multiple hydrogen atoms can be substituted

• Various termination steps can occur

Key properties of alkanes

• Only contain single bonds (sigma bonds) = saturated

• Hydrocarbons

• C_nH_2n+2

• Covalently bonded

• Simple molecular

• Relatively high bond enthalpies

• Free rotation about sigma bonds

What is the shape around each carbon in the chain?

Tetrahedral shape - all C have 4 bonded pairs of electrons around them

What is the relationship between chain length and boiling point?

As chain length increases (relative molecular mass increases) then boiling point increases

Why does boiling point increase with chain length?

• Number of electrons increases

• Greater surface points of contact

• Greater London forces

• = more energy required to overcome

What is the link between branching and boiling point of isomers?

As molecule becomes more branched, boiling point decreases

Why does boiling point decrease as isomer becomes more branched?

• Fewer surface points of contact

• Weaker London forces

• Less energy required to overcome

What is combustion?

A chemical reaction in which a substance reacts with oxygen

When does complete combustion occur?

In a plentiful supply of air

What does complete combustion of a hydrocarbon produce?

• Carbon dioxide

• Water

• + gives out energy

What is a better fuel, alkanes or alkenes?

Alkanes produce more heat energy and burn cleaner

When does incomplete combustion occur?

In a limited supply of air

What does incomplete combustion of a hydrocarbon produce?

• Water

• Carbon monoxide

• Carbon

• + less energy than complete

Mixture of carbon products are formed: CO₂, CO and C

Problems associated with incomplete combustion?

1. Carbon monoxide

2. Soot

Carbon monoxide dangers

• Toxic gas

• Odorless + colorless

• Combines with haemoglobin in red blood cells, limiting amount of oxygen transported by the blood

Soot dangers

• Particulates

• Causes breathing problems

• Blackens buildings

Hydrocarbon

A molecule containing only hydrogen and carbon

Functional group

The atom or group of atoms responsible for the characteristic reactions of a molecule

Homologous series

A series of compounds with the same functional group, each successive member differing by CH₂

• all members have the same general formula

• similar chemical properties

Aliphatic

A compound containing hydrogen and carbon joined together in straight chains, branched chains or non-aromatic rings

Alicyclic

Aliphatic compounds that contain non-aromatic rings with or without side chains

Alkyl group

A group or branch with general formula C_nH_2n+1

Aromatic

Compounds that contain. benzene a ring C₆H₆

General formula

The simplest algebraic formula of a member if a homologous series

Molecular formula

The actual number of atoms of each element present in one molecule of a compound

Empirical formula

The simplest whole number ratio of atoms of each element present in a compound

Structural formula

The minimal detail that shows the arrangement of atoms in a molecule

Displayed formula

Shows the relative positioning of all the atoms in a molecule and the bonds between them

Skeletal formula

A simplified organic formula, with hydrogen atoms removed from the alkyl chains, leaving just the carbon skeleton and associated functional groups

Three dimensional formula

This formula gives the best representation of the shape of the molecule