10.2 Functional group chemistry Hydrocarbons

Essential idea Structure, bonding and chemical reactions involving functional group inter-conversions are key strands in organic chemistry.

ALKANES General Formula: CnH2n+2 Alkanes are saturated hydrocarbons. Hydrocarbons refer to compounds that contain hydrogen and carbon only. Saturated means they contain all single carbon-carbon bonds.

LO-1 Alkanes have low reactivity and undergo free-radical substitution reactions. Alkanes have low reactivity because alkanes have Saturated nature Non-polar character

ALKANES HAVE LOW CHEMICAL REACTIVITY Alkanes contain only C-C and C-H bonds. Both of these are strong bonds with bond enthalpies of 348 kJ/mol and 412 kJ/mol, respectively. These molecules will only react in the presence of a source of energy strong enough to break these bonds. Most alkanes are stable under most conditions and can be stored, transported and compressed safely. The C-C and C-H bonds are non-polar so they are not susceptible to attack by most reactants.

Application and skills Alkanes: Writing equations for the complete and incomplete combustion of hydrocarbons. Explanation of the reaction of methane and ethane with halogens in terms of a free-radical substitution mechanism involving photochemical homolytic fission.

Combustion Complete combustion Incomplete combustion 2CH4(g) + 3O2(g)             2CO(g)    +    4H2O(l) Carbon monoxide is poisonous.

Why is Carbon Monoxide Poisonous? Carbon monoxide acts as a poison by combining with hemoglobin in the blood. Hemoglobin normally reacts with oxygen from the air and transports the oxygen to the parts of the body which need it. Carbon monoxide is much more reactive with hemoglobin than oxygen is. It combines to form a stable compound with hemoglobin, preventing the transport of oxygen around the body. The person dies by suffocating from the inside (nasty!).

Halogenations Reaction of methane with chlorine Reaction of ethane with chlorine

Homolytic Fission and Free radical substitution

Learning outcomes Understand Alkenes are more reactive than alkanes and undergo addition reactions. Application and Skills Write equations for the reactions of alkenes with hydrogen and halogens. Write equations for the reactions of symmetrical alkenes with hydrogen halides and water. Outline the addition polymerization of alkenes.

Reactions of Alkenes Hydrocarbons that have –C=C- undergo addition reactions. Addition of hydrogen ( alkene to Alkane) Reaction conditions Catalyst : Nickel( Pt or Pd) Temperature : 573 K or 300 oC 12

Reactions of Alkenes Addition of halogen to alkene

Reaction of Alkenes Reaction of Alkenes and Hydrogen halides

Reaction of Alkenes Reaction of ethene and water gives ethanol

Test to differentiate between Alkane and Alkene Alkenes decolorize bromine water Animation Video

POLYMERIZATION Alkenes readily undergo addition reactions by breaking their double bond. They can be joined together to produce long chains known as polymers. The alkene used in the reaction is known as the monomer and its chemical nature will determine the properties of the polymer. H H H H C=C has the repeating unit -C-C- H H H H n

Polymerization

ECONOMIC IMPACT Alkenes are used as starting materials in the manufacture of many industrially important chemicals. Polymers are a major product of the organic chemical industry. Most of our most common and useful plastics are polymers of alkenes.

Ethene polymerizes to form polyethene also known as polythene Ethene polymerizes to form polyethene also known as polythene. (structure shown previously) It has excellent insulating properties. It played an essential role in the development of radar during World War II. It is commonly used in household containers, carrier bags, water tanks and piping.

Propene polymerizes to form polypropene often called polypropylene. It is used in the manufacture of clothing. More specifically the production of thermal wear. H H H H C=C has the repeating unit -C-C H CH3 H CH3 n

Polychloroethene is known as PVC, poly vinyl chloride. It is one of the world’s most important plastics. It is used in construction materials, packaging, and electrical cable sheathing. Its synthesis produces dioxins which are toxic and are linked to a variety of cancers. H H H H C=C has the repeating unit -C-C - H Cl H Cl n

Polytetrafluoroethene is known as PTFE It is often marketed as Teflon. F F F F C=C has the repeating unit -C-C - F F F F n

Alkene Reaction Summary

10.2 Functional group chemistry Alcohols

Learning outcomes Understand: Alcohols undergo esterification (or condensation) reactions with acids and some undergo oxidation reactions Apply their knowledge to: Write equations for the complete combustion of alcohols. Write equations for the oxidation reactions of primary and secondary alcohol (using either acidified potassium dichromate(VI) or potassium manganate(VII) as the oxidizing agent).

Learning outcomes Apply knowledge to: Explain the importance of distillation and reflux in the isolation of the aldehyde and carboxylic acid products when primary alcohols are oxidized. Write the equation for the condensation reaction of an alcohol with a carboxylic acid, in the presence of a catalyst (e.g. concentrated sulfuric acid) to form an ester.

ALCOHOLS General Formula: CnH2n+1OH Alcohols contain the –OH functional group. The –OH group is polar and increases the solubility in water and the volatility of alcohols when compared to alkanes of similar molar mass.

COMBUSTION OF ALCOHOLS Alcohols react with oxygen to produce CO2 and H2O. These reactions produce significant amounts of energy. The amount of energy released increases as we go up a homologous series because more CO2 is produced. As in the case with hydrocarbons, in the case of limited oxygen, CO will be produced instead of CO2. 2CH3OH + 3O2 → 2CO2 + 4H2O

Esterification An alcohol reacts with an alkanoic acid to form an ester and water. 15

Oxidation Alcohols are oxidized to alkanals (aldehydes) or alkanones (ketones 9

Primary Alcohols are Oxidized to Alkanals The most common oxidizing agents are KMnO4 in basic solution, K2Cr2O7 in acidic solution, or oxygen from the air. The mechanisms of such reactions are usually complex Animation Aldehyde http://www.ibchem.com/IB16/10.33.htm Animation Carboxylic acid 10

Secondary Alcohols are oxidized to Alkanones 11

Tertiary Alcohols are not easily oxidized 12

Evidence for oxidation of alcohols Video

Halogenoalkanes

Learning outcomes Understand: Halogenoalkanes are more reactive than alkanes. Halogenoalkanes can undergo nucleophilic substitution reactions. Nucleophiles are electron-rich species containing a non-bonding pair of electrons that is donated to an electron-deficient carbon Apply their knowledge to: Write equations for the substitution reactions of halogenoalkanes with aqueous sodium hydroxide.

HALOGENOALKANES General Formula: CnH2n+1X (X=Halogen) Halogenoalkanes contain an atom of fluorine, chlorine, bromine or iodine bonded to the carbon skeleton of the molecule. Due to the presence of the halogen, they possess a polar bond unlike the non-polar alkanes.

Boiling points of some isomers Draw a conclusion about relationship between branching and boiling points of halogenoalkanes

Reactivity of halogenoalkane

Reactivity of halogenoalkanes Bond strength Vs Bond Polarity Bond strength is the winner

Nucleophiles A nucleophile is a species (an ion or a molecule) which is strongly attracted to a region of positive charge in something else. Nucleophiles are either fully negative ions, or else have a strongly - charge somewhere on a molecule. Common nucleophiles are hydroxide ions, cyanide ions, water and ammonia.

The nucleophiles are attracted to the electron deficient carbon in the halogenoalkane which leads to a reaction where substitution of the halogen occurs. This reaction is called Substitution Nucleophilic and has the shorthand notation of SN. During this reaction, the carbon-halogen bond breaks and the halogen is released as a halide ion. When both of the shared electrons go to one product, it is known as heterolytic fission. The halogen that becomes detached is sometimes referred to as the leaving group. C2H5Br + OH- → C2H5OH + Br-

Nucleophilic substitution reactions. Halogenoalkanes undergo nucleophilic substitution reaction Simples way to write

OH1- as Nucleophile The halogenoalkane is usually refluxed with aqueous sodium hydroxide, NaOH(aq), but some RX molecules are reactive enough to hydrolyse when just mixed with water

Benzene

Learning outcomes Understanding Benzene does not readily undergo addition reactions but does undergo electrophilic substitution reactions.

Reactivity of benzene Benzene does not readily undergo addition reaction due to delocalized bonding Benzene undergo electrophilic substitution reaction

Benzene and electrophiles Because of the delocalised electrons exposed above and below the plane of the rest of the molecule, benzene is obviously going to be highly attractive to electrophiles - species which seek after electron rich areas in other molecules The electrophile will either be a positive ion, or the slightly positive end of a polar molecule. Benzene undergo substitution reaction because it needs high energy to break delocalized electron cloud.

Examples of substitution reactions Nitration Chlorination