Chemistry

Unit 2

Representing Organic Compounds

The empirical formula is the simplest whole-number ratio of atoms of each element present in a compound.

The molecular formula is the actual number of atoms of each element in a molecule.

The general formula is an algebraic formula for a member of a homologous series, for example, CnH2n+2 for an alkane.

Structural, displayed and skeletal formulae are all used to show organic molecules in different ways.

Isomerism

Structural isomers are compounds with the same molecular formula, but different structural formulae.

Stereoisomers are compounds with the same structural formula, but with a different arrangement in space.

E/Z isomerism is an example of stereoisomerism in which lack of free rotation about the C=C bond allows two different forms to exist.

Cis-trans isomerism occurs when two hydrogen atoms, on different carbon atoms, adopt either the same or opposite sides of the C=C.

Hydrocarbons from Crude Oil

A hydrocarbon is a compound containing hydrogen and carbon atoms only.

Crude oil is separated by fractional distillation, in which the fractions have different boiling points.

The carbon atom in hydrocarbons, as in alkanes, is forming four single covalent bonds in a tetrahedral arrangement.

As the chain length of the hydrocarbons increases, so too does the size of the van der Waals’ force, resulting in a high boiling point.

Reactions of Alkanes

Alkanes are hydrocarbons, and their molecules are weakly attracted by van der Waals’ forces.

Alkanes are saturated hydrocarbons; this means that they consist of only carbon single bonds.

Alkanes are mainly used as fuels, when combusted they provide heat energy.

Alkanes react with halogens in ultraviolet light to form halogenoalkanes. This is a free radical substitution mechanism.

Alkenes

Alkenes are unsaturated hydrocarbons; they consist of one or more carbon double bonds, making them more reactive than alkanes.

A double bond consists of a s- and a p-bond. The p-bond is formed by the adjacent overlap of p-orbitals.

The p-bond is the reactive part of the double bond; the s-bond is of lower reactivity.

The arrangement of the bond about the carbon double bond is trigonal planar, with an internal angle of 120°.

Chemical reactions of the alkenes

Alkenes react with hydrogen (to form alkanes); with halogens (to form dihalogenoalkanes); and steam (to form alcohols).

Alkenes also react with hydrogen halides (to form halogenoalkanes) and can polymerise to form addition polymers, e.g. polypropene.

The reaction between a halogen and an alkene involves an electrophilic addition mechanism.

An electrophile is a lone pair electron acceptor. Be able to draw the mechanisms!

Properties and preparation of ethanol

Alcohols consist of one or more –OH groups in an organic molecule.

Alcohols are all water soluble and have relatively high boiling points (are not very volatile) as a result of hydrogen bonding.

Ethanol can be made industrially by the reaction of ethane with steam in the presence of a phosphoric(V) acid catalyst.

Ethanol for the drinks industry is produced by the fermentation of sugars, such as glucose.

Reactions of alcohols

Alcohols may be classed as either primary, secondary or tertiary.

Primary alcohols may be oxidised to form aldehydes then carboxylic acids, and secondary alcohols to form ketones.

Alcohols may react with carboxylic acids, in the presence of a concentrated sulfuric(VI) acid catalyst, to form esters.

Alcohols may eliminate water, in the presence of an acid catalyst, to form alkenes.

Halogenoalkanes

Halogenoalkanes are formed when a hydrocarbon has one or more hydrogen atoms substituted for halogen atoms.

Halogenoalkanes may be hydrolysed to form alcohols, using hot sodium hydroxide solution.

The hydroxide ion acts as a nucleophile when reacting with a halogenoalkane. A nucleophile is a lone pair electron donor.

The C–I bond is weaker than the C–Br and C–Cl bond, and so it breaks faster when attacked by nucleophiles.

Uses of halogenoalkanes

Chloroethene and tetrafluoroethene are used to make the polymers PVC and PTFE (Teflon) respectively.

CFCs (chlorofluorocarbons) were manufactured for use in aerosols and refrigerants because of their low reactivity and high volatility.

CFCs have caused the ozone layer to break down, leading to an increased intensity in damaging ultraviolet radiation on Earth.

Biodegradable CFCs and HCFCs are now used as alternatives to CFCs.

Infrared spectroscopy and mass spectroscopy

Infrared spectroscopy is used to detect the presence of certain covalent bonds in an organic molecule by bond vibration.

An absorption of approximately 3000 cm-1 appears in most organic molecules, as this relates to the C-H bond.

Mass spectrometry can be used to detect the presence of isotopes and their abundances.

When an organic molecule is placed into a mass spectrometer, fragments may appear, such as C2H5+ at m/z = 29.

Enthalpy changes

Exothermic reactions are those that produce heat energy, while endothermic reactions absorb heat energy.

Standard enthalpy of formation is the heat change involved when one mole of a compound is formed from its elements under standard conditions.

Standard enthalpy of combustion is the heat released when one mole of a compound is combusted in excess oxygen under standard conditions.

Average bond enthalpy is the heat energy required to break one mole of the specified bonds in the gas phase.

Rates of reaction

The collision theory enables us to explain the effect of changing concentration, surface area, temperature etc on reaction rates.

For a reaction, collisions must take place between particles with sufficient energy before a successful collision may result.

A catalyst works by increasing the rate of a chemical reaction, at the end of which the catalyst is chemically unchanged.

A catalytic converter allows pollutant gases like CO and NO to react on the surface of Pd, Pt and Rh to form less harmful products.

Effect of temperature on reaction rate

Particles at a fixed temperature: a range of energies – some have low and some high, but most have a middling energy.

Reactions have a certain minimum energy requirement for a reaction to take place; this is called the activation energy.

As temperature increases, the distribution moves to the right, giving a greater proportion of molecules exceeding the activation energy.

As more particles have an energy exceeding the activation energy, there will be a greater chance of a successful collision.

Dynamic Equilibrium

In a reaction at equilibrium, the forward and reverse reaction rates are equal.

Le Chatelier’s principle: an equilibrium will oppose a change in external conditions by shifting to reduce the effect of the change.

In the Haber process, a compromise temperature is used between the rate and yield for ammonia.

A greater yield of ammonia is formed at high pressure due to fewer gas molecules existing on the product side of the equilibrium.

Chemistry of the air

The greenhouse effect results from certain gases in the atmosphere, such as carbon dioxide, absorbing infrared radiation.

Bonds like C=O, O–H and C–H bonds in carbon dioxide, water and methane absorb infrared radiation.

Ozone in the stratosphere absorbs ultraviolet radiation from the Sun and oxygen is formed. This is a reversible process.

Air pollution can be controlled by reducing hazardous chemicals from industry and using high atom economy processes.