[3] Properties of Alkanes

What is the general formula of alkanes?

CₙH₂ₙ₊₂.

What are the key properties of members of a homologous series?

They have the same functional group, similar chemical properties, and successive members differ by CH₂.

What is a sigma (σ) bond?

A covalent bond formed by direct overlap of orbitals between the bonding atoms, with electron density concentrated along the line between the nuclei.

Why can rotation occur around a C–C sigma bond?

The electron density in a sigma bond lies along the axis between the two carbon atoms, allowing the atoms to rotate around the bond.

What is the shape around each carbon atom in an alkane?

Tetrahedral.

What is the bond angle around each carbon atom in an alkane?

109.5°.

Why is the carbon atom in an alkane tetrahedral?

Each carbon atom is surrounded by four bonding electron pairs. Repulsion between these electron pairs causes them to arrange themselves as far apart as possible in a tetrahedral arrangement.

Why is the bond angle in an alkane 109.5°?

Four bonding electron pairs repel each other and arrange themselves as far apart as possible, giving a tetrahedral arrangement with bond angles of 109.5°.

What is a hatched wedge used to show?

A hatched wedge shows a bond going into the plane of the paper.

What does a normal solid line show in a 3D structure?

A bond lying in the plane of the paper.

Are alkane structures rigid?

No. The sigma bonds act as axes around which atoms can rotate freely, so alkane molecules can adopt different shapes.

What is a conformational change in an alkane?

A change in the shape of an alkane caused by rotation around a C–C sigma bond.

What are London forces?

Attractive forces between induced dipoles in neighbouring molecules.

Where are London forces present?

Between all molecules, including both polar and non-polar molecules.

How does a temporary dipole form?

At any instant, electrons may be unevenly distributed within an atom or molecule, producing a temporary dipole.

What is an instantaneous dipole?

A temporary uneven distribution of electron density caused by electrons being more concentrated on one side of an atom or molecule.

How does an induced dipole form?

An instantaneous dipole in one particle repels or attracts electrons in a neighbouring particle, causing an uneven electron distribution and producing an induced dipole.

How do London forces arise?

An instantaneous dipole forms in one molecule and induces a dipole in a neighbouring molecule. The opposite partial charges attract, producing a London force.

What is the exam definition of a London force?

The attraction between opposite temporary dipoles on neighbouring molecules.

Why are London forces temporary?

The dipoles are caused by constantly changing electron distributions, so the dipoles continually form and disappear.

Why can London forces still produce significant attraction?

Although each individual dipole is temporary, many temporary induced dipole interactions occur between molecules over time.

What happens to London force strength as the number of electrons increases?

London forces become stronger because molecules with more electrons can form larger instantaneous and induced dipoles.

What does polarisable mean?

A molecule is more polarisable when its electron cloud can be distorted more easily.

Why are larger molecules generally more polarisable?

They contain more electrons and their outer electrons are further from the nucleus, so the electron cloud can shift more easily.

What happens to London forces as molecular size increases?

The London forces generally become stronger because there are more electrons and greater polarisability.

What happens when an alkane is heated from liquid to gas?

Thermal energy is supplied to overcome the London forces between alkane molecules, allowing the molecules to separate and enter the gas state.

What holds alkane molecules close together in the liquid state?

Weak attractive London forces between the molecules.

What happens to London forces during boiling?

Sufficient thermal energy is supplied to overcome the intermolecular London forces, allowing the molecules to separate.

Why does the boiling point increase as carbon chain length increases?

As carbon chain length increases, the surface area of the molecule increases. This increases the number of contact points between adjacent molecules, increasing the number and strength of London forces. More heat energy is therefore needed to overcome these forces.

What is the full exam scaffold for explaining increasing boiling points with chain length?

Increasing carbon chain length → increased surface area → more contact points between adjacent molecules → stronger/more London forces → more heat energy needed to overcome the forces → higher boiling point.

Why does greater surface area increase London forces?

A larger surface area allows more contact between neighbouring molecules, increasing the number of London force interactions.

What happens to the boiling point of alkanes when branching increases?

The boiling point decreases.

Why do branched alkanes have lower boiling points than unbranched alkanes?

Branching decreases the surface area and means the molecules cannot fit as closely together, giving fewer points of contact between molecules. Therefore there are fewer/weaker London forces and less heat energy is needed to overcome them.

What is the full exam scaffold for explaining the effect of branching on boiling point?

Increased branching → decreased surface area → fewer points of contact between adjacent molecules → fewer/weaker London forces → less heat energy needed to overcome the forces → lower boiling point.

Why does butane have a higher boiling point than 2-methylpropane?

Butane is unbranched and has a larger surface area and more points of contact between molecules.

Therefore it has more London forces, requiring more heat energy to overcome them, giving it a higher boiling point.

Why do structural isomers of an alkane have different boiling points?

Different structures have different surface areas and numbers of points of contact, which affects the strength and number of London forces between molecules.

Why does 2-methylbutane have a lower boiling point than pentane?

2-methylbutane is branched, so it has a smaller surface area and fewer points of contact between molecules. Therefore it has fewer/weaker London forces and requires less heat energy to overcome them.

What are the boiling points of pentane and 2-methylbutane?

Pentane: 36 °C; 2-methylbutane: 27 °C.

Why does the boiling point increase from ethane to pentane?

Increasing carbon chain length increases surface area and the number of contact points between molecules, increasing the strength/number of London forces. More heat energy is therefore needed to overcome these forces.

What are the five structural isomers of C₆H₁₄?

Hexane, 2-methylpentane, 3-methylpentane, 2,2-dimethylbutane and 2,3-dimethylbutane.

Why does hexane have the highest boiling point of the C₆H₁₄ isomers?

Hexane is unbranched, so it has the largest surface area and most surface interactions/points of contact between adjacent molecules. Therefore it has the strongest overall London forces and requires more heat energy to overcome them.

Why does 2,2-dimethylbutane have the lowest boiling point of the C₆H₁₄ isomers?

It is the most highly branched, giving it the smallest effective surface area and fewest points of contact between molecules. Therefore it has fewer/weaker London forces and requires less heat energy to overcome them.

How does increasing branching affect surface area in alkane isomers?

Increased branching decreases the effective surface area available for contact between molecules.

How does decreasing surface area affect London forces?

It decreases the number of contact points between molecules, so there are fewer London force interactions.

How does branching explain the boiling-point trend across C₆H₁₄ isomers?

As branching increases, surface area decreases, reducing the number of points of contact and London forces. Less heat energy is needed to overcome the forces, so the boiling point decreases.

What are the key factors affecting London force strength in alkanes?

The number of electrons/molecular size, chain length and branching/surface area.

What happens to the electron cloud in a more polarisable molecule?

It can be distorted more easily, allowing larger instantaneous and induced dipoles to form.

What is the relationship between number of electrons and polarisability?

More electrons generally means greater polarisability because the electron cloud is easier to distort.

What is the relationship between polarisability and London force strength?

Greater polarisability allows larger temporary/induced dipoles, resulting in stronger London forces.

What must you mention when explaining boiling point differences between alkane isomers?

Surface area, points of contact, London forces and the amount of heat energy needed to overcome those forces.

Why do alkanes have relatively low boiling points compared with substances with stronger intermolecular forces?

The intermolecular forces between alkane molecules are London forces, which are relatively weak.

What happens to the intermolecular forces when thermal energy is increased?

The molecules gain kinetic energy and can overcome the attractive London forces, allowing them to move further apart.

What is the key difference between intramolecular sigma bonds and intermolecular London forces?

Sigma bonds are strong covalent bonds within molecules, whereas London forces are weak intermolecular attractions between molecules.

What are the three OCR A learning objectives for properties of alkanes?

Know alkanes are saturated hydrocarbons containing sigma bonds with free rotation; explain the tetrahedral shape and 109.5° bond angle using electron pair repulsion; explain boiling-point variations with chain length and branching using London forces.