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.

What are alkenes?

Unsaturated hydrocarbons with C=C bonds comprising a pi-bond above & below the bonding of C atoms, & a sigma-bond

What are stereoisomers?

Compounds with the same molecular formula but with a different arrangement in space

What is the structure of an alkene?

Trigonal planar shape, bond angle of 120

What is E/Z isomerism?

An example of stereoisomerism:

• C=C bond restricts rotation

• Requires 2 different groups to be attached to each C atom of the C=C group

E = higher priority groups on EITHER side of C=C bond

Z = higher priority groups on SAME side of C=C bond

What is cis-trans isomerism?

A special case of E/Z isomerism in which 2 substituent groups attached to each of the C atom of the C=C group are the same

How does hydrogenation occur in alkenes?

Addition reaction of hydrogen to an alkene in the presence of a nickel catalyst, to form alkanes

How does hydration occur in alkenes?

Addition reaction of steam (H2O) to an alkene in the presence of a phosphoric acid catalyst, to form alcohols

How do you test for unsaturation in a carbon chain?

Reacting an alkene with a halogen e.g. Br to detect the presence of a double C=C bond

What is an electrophile?

An electron pair acceptor

What are the types of products of organic reactions and why are they formed?

Minor

Major - formed via the more stable carbocation

How can the combustion (burning) of waste polymers be beneficial for sustainability?

Waste polymers can be incinerated to produce heat, generating steam to drive a turbine, producing electricity (energy production)

How can feedstock be beneficial for sustainability?

Waste polymers can be broken down by chemical or thermal processes, into monomers, gases or oils.

→These products are used as raw materials in the production of new polymers & organic chemicals

How can waste polymers remove toxic waste products?

• Toxic HCl gas is produced by the combustion of PVC

• Sodium hydrogencarbonate can be used to neutralised HCl gas

How are biodegradble polymers beneficial to the environment?

They can be broken down by microbes/microorganisms over time

How are photodegradable polymers beneficial to the environment?

They contain bonds that are weakened by absorbing light, so they can break down