4.1 Basic Concepts & Hydrocarbons (Organic Chemistry)

What is a general formula?

The simplest algebraic formula of a member of a homoglous series e.g. for an alkane: CnH2n+2

What is a structural formula?

The minimal detail that shows the arrangement of atoms in a molecule) e.g. for butane: CH3CH2CH2CH3 or CH3(CH2) 2CH3

How are carboxyl & ester groups represented in structural formulae?

• Carboxyl group = COOH

• Ester group = COO

What is a displayed formula?

The relative positioning of atoms and the bonds between them

What symbols are used for cyclohexane & benzene?

Cyclohexane & benzene are cyclic

• Cyclohexane is drawn as a hexagon

• Benzene is a hexagon with double bonds/circle inside

What is a skeletal formula?

The simplified organic formula, shown by removing hydrogen atoms from alkyl chains, leaving just a carbon skeleton and associated functional groups

Define molecular formula

The exact number of atoms of each element present in the molecule e.g. C4H10

Define empirical formula

The simplest whole number ratio of atoms in the molecule e.g. C2H5

Define homoglous series

A series of compounds of similar structure in which each member differs from the next by a common repeating unit, CH2

What is the difference between saturated and non-saturated?

Saturated: only containing single C-C bonds

Unsatured: presence of multiple carbon–carbon bonds, including C=C, C(triple)C / and aromatic rings

What is an aliphatic compund?

A compound containing C & H joined together in straight chains, branched chains, or non-aromatic rings

What is an alicyclic compound?

An aliphatic compound arranged in non-aromatic rings with or without side chains

What is an aromatic compound?

A compound that contains a benzene ring

Define structural isomers

Compounds that have the same molecular formula but different structural formula

What is homolytic fission?

Breaking a covalent bond so that each atoms takes one electron from the bond, forming 2 radicals

What is heteroytic fission?

Breaking a covalent bond so that the more electronegative atoms takes both electrons from the bond, forming a negative & positive ion

Define radical

A species with an unpaired electron (represented by dots)

What is an alkane?

A saturated hydrocarbon containing single C-C & C-H bonds

What is a tetrahedral shape?

• 4 bonded pairs

• Bond angle of 109.5^o

• Electron pairs positioned as far apart as possible due to repulsion

What causes variations in boiling points in alkanes?

• Branching

• No. of C atoms

How does branching affect the boiling point of allkanes?

More branching = Lower boiling point

• Fewer points of surface contact between molecules

• Fewer/weaker London forces → less energy to overcome

• Branching prevents molecules from being compact

How does the number of C atoms affect boiling point in alkanes?

More C atoms = Higher boiling point

• More points of surface contact

• Greater induced dipole-dipole interactions → more energy to overcome

Why do alkanes have low reactivity?

High bond enthalpy

• C-H bond is stronger than C-C bond as its bond length is shorter

• H only consists of one shell, so distance between nuclei is shorter → greater force of attraction to nuclei & shared pair of electrons 

• This gives σ-bonds high bond enthalpy (than π bonds

• Makes bonds difficult to break, so low reactivity

Low polarity 

• Electronegativites between C & H are very similar → σ- bonds have low polarity

What happens in combustion of alkanes?

Complete Combustion:

• In plentiful supplies of O₂ they give CO₂ & H₂O

• CₓHᵧ + (x + ʸ/₄)O₂ → xCO₂ + ʸ/₂(H₂O)

Incomplete Combustion

• Occurs in limited O₂ supply

• Produces CO & H₂O

What are the dangers of carbon monoxide (CO)?

CO is colourless, odourless & slightly toxic.

• It’s better at binding to haemoglobin than O₂

• This could lead to suffocation

How do alkanes react with chlorine & bromine?

Through free-radical substitution, requiring UV light.

What are the limitations of free-radical substitution?

Further substitution occurs, producing a variety of products.

Substitution can occur at different positions in the carbon chain.