Topic 10 - Organic Chemistry

Functional groups soluble in water

- Alcohols - Carboxylic acids - Amines (primary and secondary) - Aldehydes - Ketones - Amides (primary and secondary) - Esters - Halogenoalkanes (unsymmetrical in shape)

Functional groups from most volatile to least volatile

- Alkane - Halogenoalkane - Aldehyde - Ketone - Amides - Amines - Alcohol - Carboxylic acid

Influence of the hydrocarbon skeleton on volatility of compounds

- Higher members of each homologous series have larger molecules and therefore stronger Van der Waals' forces between them, thus having a higher boiling point. - Branching of the hydrocarbon chain lowers the strength of the intermolecular forces, thus lowering the boiling point of the compound.

Economic importance of the reaction of alkenes

- Hydrogenation is used in the margarine industry. - The hydration of ethene is important in the manufacture of ethanol. - Polymerization is an important process since it is used to manufacture plastics.

Hydrogenation is used in the margarine industry

- It is used to convert vegetable oils containing many unsaturated hydrocarbon chains into margarine, which is made up of more saturated hydrocarbons. - This causes for margarine to have a higher boiling point and therefore for it to be solid at room temperature.

Examples of physical properties

- Mass - Density - Boiling point - Melting point - Viscosity (how thick/fluid a substance is) - Flammability (ability to ignite) - Rate of diffusion (for gases only)

Examples of substitution reactions

- Reaction of methane and chlorine to produce chloromethane and hydrogen chloride. - Reaction of ethane with bromine to produce bromoethane and hydrogen bromide.

Features of a homologous series

- Successive members differ by a CH2 group. - All compounds follow the same general formula. - The compounds have similar chemical properties, since they have the same functional groups and therefore the same reactivity. - Successive members show a gradation in their physical properties.

SN1 mechanism

- The negative species is repelled by the bulky electron clouds of the substituent groups. So, the halogen will first lose the halogen through heterolytic fission and form a carbocation. - The carbocation formed is stabilized by the positive inductive effect of the alkyl groups. - The nucleophile attacks the carbocation to form the product of the reaction.

SN2 mechanism

- The nucleophile attacks the halogenoalkane, displacing the halogen and taking its position in the molecule. - Heterolytic fission of the carbon-halogen bond takes place.

Steps to naming isomers of alkanes

1) Identify the longest carbon chain. 2) Identify any side chains/alkyl groups and their position. eg. 2-methylpropane is an isomer of butane since it has 4 carbon atoms.

Apply IUPAC rules for naming compounds containing up to six carbon atoms, with one functional group.

1) Identify the longest straight chain of (continuous/unbranched) C atoms. 2) Identify the functional group and its position. 3) Identify any side chains or substituent groups and their positions. Substituents are given in alphabetical order.

Substitution reaction mechanism

1) Initiation (1 step) 2) Propagation (2 steps or more) 3) Termination (1 step for each product)

Nucleophile

A molecule/species/reactant that has a lone pair of electrons. Examples: - Bromide ion - Water - Ammonia - Hydroxide ion (from KOH or NaOH)

Identify primary, secondary and tertiary carbon atoms in alcohols and halogenoalkanes.

A primary carbon atom is attached to at least two hydrogen atoms and to the functional group, resulting in a primary molecule. A secondary carbon atom is attached to the functional group and is also bonded to two alkyl groups, resulting in a secondary molecule. A tertiary carbon atom is attached to the functional group and is also bonded to 3 alkyl groups, thus not being bonded to any hydrogen atoms. Molecules with this arrangement are known as tertiary molecules.

Skeletal formula

A way of representing a molecule in which each corner represents a carbon atom and the presence of H atoms is generally assumed.

Why is it that you can distill an aldehyde off as it forms?

Aldehydes have lower boiling points than both alcohols and carboxylic acids since they are unable to form hydrogen bonds between molecules. The aldehyde will turn into a gas more readily and it can then be condensed in order to isolate it from the reaction mixture.

Explain the low reactivity of alkanes in terms of bond enthalpies and bond polarity.

Alkanes contain only C-C and C-h bonds. These are strong bonds with a high bond enthalpy, meaning that these molecules will only react in the presence of a strong source of energy such as UV light. These bonds are essentially non-polar and their low polarity results in the molecule being neutral. Hence, it will not easily be attracted to other molecules.

Distinguish between alkanes and alkenes using bromine water.

Alkenes and alkanes are both colourless, whereas bromine water has an orange colour. When separate samples of an alkene and an alkane are shaken together with bromine water at room temperature: - The alkane will turn the same colour as the bromine water since no reaction will take place, as alkanes are very unreactive. - The bromine water is quickly decolorized by the alkene since an addition reaction will take place, making a colourless dibromoalkane.

Functional group

An atom or group of atoms responsible for the characteristic reactions of a molecule/homologous series. The activity of a functional group is often influences by its position in the carbon chain.

Conditions required for an oxidation reaction to take place

An oxidizing agent is required, such as acidified potassium dichromate (VI). The mixture must be heated under reflux.

Trends in boiling points of members of a homologous series

As the molar mass of a compound increases, so does its melting and boiling point since there are stronger Van der Waals' forces between molecules. As you go down a homologous series, there is a greater molar mass since a CH2 unit is added each time. Thus, the boiling point increases due to stronger intermolecular forces. NOTE: As the length of the chain increases, the percentage change in molar mass becomes progressively smaller.

Outline the addition polimerization of alkenes.

Because alkenes readily undergo addition reactions by breaking their double bonds, they can be joined together to produce long chains called addition polymers. The alkene used in this reaction is known as the monomer.

SN2

Bimolecular nucleophilic substitution. Occurs with primary halogenoalkanes. The rate is proportional to the concentration of the halogenoalkane and to the concentration of the nucleophile, as both are involved in the rate-determining step.

Condensed structural formula

Can omit bonds between atoms and can show identical groups bracketed together. R can be used to represent an alkyl group and a ring can be used to represent benzene.

Carbon monoxide problems

Carbon monoxide is a toxin (toxic substance)/poisonous. It combines irreversibly with the haemoglobin in the blood and prevents it from carrying/transporting oxygen.

Polymerization of chloroethene

Chloroethene polymerizes to form polychloroethene/polyvinylchloride (PVC). It is very widely used in all forms of construction materials, packaging, electrical cable sheathing and so on.

Complete and incomplete combustion

Complete combustion occurs when there is excess oxygen and it releases carbon dioxide and water. Incomplete combustion occurs when there is a limited amount of oxygen and it releases carbon particles (soot), carbon monoxide and water.

Structural isomers

Compounds with the same molecular formula but with a different structural formula/arrangement of atoms within the molecule. Each isomer is a distinct compound, having its own unique physical and chemical properties.

Apply IUPAC rules for naming the isomers of the straight-chain alkenes up to C6.

Double-blonds are identified by the lowest number of the carbon atoms they are attached to.

The hydration of ethene is important in the manufacture of ethanol

Ethanol is a very important solvent and can be manufactured on a large scale from ethene. Ethanol is not only used as a fuel, but it is commonly found in alcoholic beverages, amongst others.

Polymerization of ethene

Ethene polymerizes to form polyethene/polythene. Polyethene is commonly used in the manufacture of household containers, carrier bags, water tanks and piping.

Homologous series

Families of compounds

Order of reactivity of hydrogen halides

HI > HBr > HCl This is because HI has a lower bond enthalpy so it reacts more readily.

Functional groups from most soluble in water to least soluble in water

Highly soluble since they are polar and can form hydrogen bonds with water: - Alcohols - Carboxylic acids - Amines (primary and secondary) Soluble due to polarity: - Aldehydes - Ketones - Amides (primary and secondary) - Esters Insoluble: - Alkanes - Alkenes The solubility of halogenoalkanes depends highly on the elements present and the way they are bonded together.

Volatility

How easy it is for molecules to turn into a gas. If a compound has a low volatility, a lot of energy is required to turn it into a gas, meaning that it has a high boiling point. Volatility is determined by a molecule's intermolecular forces.

Obtaining a high yield of the products of the oxidation of primary alcohols

If we want to obtain a high yield of the aldehyde: - Distill the product off as it forms. - Limit the amount of oxidizing agent added. If we want to obtain a high yield of the carboxylic acid: - Heat the mixture under reflux. - Leave the mixture in contact with excess oxidizing agent for a prolonged period of time.

The number of isomers that exist for a molecular formula increases with the size of the molecule/with the number of atoms in the molecule.

Methane, ethane and propane have no structural isomers since the carbon chains are too short. Butane has two structural isomers. Pentane has three structural isomers. Hexane has five structural isomers.

Monomers and polymers

Monomers are smaller molecules with a lower molecules mass and therefore less strong intermolecular forces between molecules. They are generally gases. Polymers are generally solids since they have a higher Mr and stronger intermolecular forces.

Initiation

Occurs only in the presence of ultra-violet light since alkanes are very unreactive. There is the homolytic fission of the halogen molecule into free radicals, which have an unpaired electron.

Heterolytic fission

Occurs when a bond is broken in a way that there is an uneven distribution of the electrons in the bond. One atom keeps both electrons and the other atom keeps none.

Homolytic fission

Occurs when the bond between two atoms is broken in a way that splits the shared pair of electrons between the two atoms. "Homo" means "the same" and refers to the fact that the two products have an equal assignment of electrons from the bond.

Propagation

Once the free radicals have been formed in initiation, they start a chain reaction in which a mixture of products is formed. The molecules react with free radicals. These reactions use and produce free radicals, allowing for the reaction to continue. Different products may result due to the different combinations of radicals and molecules.

Influence of the functional group on the volatility of compounds

Organic compounds my have other intermolecular forces other than Van der Waals' forces depending on the nature of their functional groups. Polar compounds develop dipole-dipole interactions with neighbouring molecules, which raises their boiling points. Polar compounds that are able to form hydrogen bonds between molecules will have even higher boiling points, as these bonds are even harder to break.

Oxidation of primary alcohols

Primary alcohols are oxidized in a two-step reaction. First, an aldehyde is formed and then it is oxidized further into a carboxylic acid.

Polymerization of propene

Propene polymerizes to form polypropene/polypropylene. This polymer is often used in the manufacture of clothing, especially thermal wear for outdoor activities.

Alkanes

Saturated hydrocarbons (all carbon atoms make four bonds).

Oxidation of secondary alcohols

Secondary alcohols are oxidized to a ketone.

Substitution reactions of secondary halogenoalkanes

Secondary halogenoalkanes can undergo SN1 or SN2 reactions.

Full structural formula

Shows every bond and atom, usually with 90º or 180º angles.

Empirical formula

Shows the simplest whole number ratio of the atoms in a molecule.

Molecular formula

Tells us the actual number of atoms of each element present in a molecule.

Oxidation of tertiary alcohols

Tertiary alcohols are not readily oxidized under comparable conditions.

Solubility

The ability of a substance (solute) to dissolve in a solvent.

Colour change observed in oxidation reactions of alcohols

The acidified potassium dichromate (VI) oxidizing agent used will change colour from orange to green when an alcohol is oxidized. Hence, there will be a colour change with primary and secondary alcohols but not with tertiary alcohols.

Positive inductive effect

The central carbon atom attracts the electrons of the molecular clouds of the alkyl groups bonded to it so that it is stabilized in a way that allows for it to hold its positive charge.

Why do addition reactions occur with alkenes?

The double bond is the site of reactivity of the molecule. Since the pi bond is a weaker bond, it is relatively easy to break and this creates two new bonding positions on the carbon atoms.

A substitution reaction cannot take place in the dark.

The energy of UV light is necessary to break the covalent bond in the halogen molecule.

Termination

The free radicals react with each other and pair up their electrons to form a variety of different products, thus removing free radicals from the mixture. A variety of products can be made, although some are more common than others.

Influence of the hydrocarbon skeleton on solubility of compounds in water

The longer the hydrocarbon skeleton, the less polar the molecule will be and therefore the less soluble in water it will be. Higher members of all homologous series are less soluble in water than lower members since they have a longer hydrocarbon chain.

Hydration

The process by which an alkene reacts with water to form an alcohol. This reaction requires concentration sulfuric acid as a catalyst, together with steam and heat.

Dihalogenation

The process by which halogens react with alkenes to produce dihalogenoalkanes. These reactions happen quickly at room temperature and are accompanied by the loss of colour of the reaction halogen.

Halogenation

The process by which hydrogen halides (such as HCl or HBr) react with alkenes to produce halogenoalkanes. These reactions take place rapidly in solution at room temperature.

Hydrogenation

The process by which hydrogen reacts with alkenes to form alkanes. This is an addition reaction in which Nickel (Ni) is used as a catalyst. This reaction occurs at high temperatures.

Monomer

The repeating unit of a polymer. Its chemical properties determine the chemical properties of the polymer.

Nucleophilic substitution

The replacement of an atom or a group of atoms in a molecule by a species with a non-bonding (lone) pair of electrons.

Structural formula

The unambiguous representation of a molecule, showing how the atoms are arranged/bonded to each other.

Halogenoalkanes undergo nucleophilic substitution reactions with sodium hydroxide.

There are different mechanisms of nucleophilic substitution, depending on the type of halogenoalkane and on molecularity.

Problems with unburned carbon particulates (soot)

They cause respiratory problems. they contribute towards global dimming, a serious environmental problem.

The main type of reaction undergone by alkanes is substitution.

This occurs when another reactant takes the place of an atom in an alkane.

Dimmer

Two units of a polymer

SN1

Unimolecular nucleophilic substitution. Occurs with tertiary halogenoalkanes. The rate is proportional to the concentration of the halogenoalkane only, since it is involved in the rate-determining step.

Alkenes

Unsaturated hydrocarbons, containing a double bond between carbon atoms.