Chpt. 23: Types of Reactions In Organic Chemistry

So far in organic chemistry we have studied fuels and the various families of organic compounds. In this final organic chapter we will study some of the reactions that these organic families undergo!!!!!

This chapter involves a study of the : 1) various types of chemical reactions that organic compounds undergo 2) synthesis of organic compounds 3) methods used to analyse for the presence of various organic substances

Part1: Types of chemical reactions that organic compounds undergo

Organic reactions can be classified as follows: - Substitution Reactions - Addition Reactions - Polymerisation Reactions - Elimination Reactions - Redox Reactions - Reactions as Acids

Substitution Reactions

Apart from combustion alkanes are rather unreactive. However, in addition to combustion reactions ALKANES also undergo SUBSTITUTION reactions Definition: A substitution reaction is a chemical reaction in which an atom or group of atoms in a molecule is replaced by another atom or group of atoms.

Types of Substitution Reactions: - Halogenation of Alkanes (Free Radical Substitution) - Esterification (previously discussed) - Saponification (previously discussed)

Halogenation of Alkanes (Halogen replaces H): Ordinary Level This involves the reaction of the alkanes with halogens in the presence of light e.g. 1)Methane reacts with chlorine in the presence of ultraviolet light to yield (mono)chloromethane and HCl: CH 4 + Cl 2 → CH 3 Cl + HCl *Space for diagram

2)Ethane reacts with chlorine in the presence of ultraviolet light to form (mono)chloroethane: C 2 H 6 + Cl 2 → C 2 H 5 Cl + HCl *Space for diagram

Higher Level Must understand the mechanism of this reaction i.e. the detailed step by step description of how the overall reaction occurs. As discussed preciously ( chemical bonding, thermochemistry) chemical reactions involve the breaking and forming of bonds. When such bonds are broken and formed it involves the rearrangement of electrons and this is what needs to be considered when studying reaction mechanisms.

Reaction Mechanism Halogenation STAGE 1: INITATION (Getting it started) - UV light breaks down the chlorine molecule into two chlorine atoms – HOMOLYTIC FISSION Cl Cl → Cl + Cl - this is called a photochemical reaction i.e. a rxn that is brought about by light. This is proven to be a light dependent reaction as it does not occur in the dark at room temperature Covalent BondUnpaired electron uv

STAGE 2: PROPAGATION 1 (Keep it going) - a free chlorine atom attacks a methane molecule to form HCl and a methyl free radical (Free radical is any atom or group of atoms with an unpaired electron) Cl + CH 4 → HCl + CH 3 - due to an incomplete outer shell the methyl free radical is very reactive Unstable

STAGE 3: PROPAGATION 2 (Keep it going) - this methyl free radical attacks a chlorine molecule to form chloromethane and a free Cl atom: CH 3 + Cl 2 → CH 3 Cl + Cl - the chlorine atom produced during propagation 2 can then react with another molecule of methane as in propagation 1: - thus a chain reaction is started!!!!!! Cl + CH 4 → HCl + CH 3

- Evidence for this chain reaction - chemists compared the amount of light falling on this reaction with the amount of light emitted. They were then able to determine the number of photons that were absorbed by the reactants and found that thousands of molecules of chloromethane were produced for every photon that was absorbed.

STEP 4: TERMINATION (Grinding to a Halt) - The chain reaction comes to an end when – the various free radicals combine with each other to form un-reactive molecules i.e. (not free radicals) - Some possible combinations (Chlorine & Methane): CH 3 + Cl → CH 3 Cl Formation of chloromethane Cl + Cl → Cl 2 Formation of chlorine molecule CH 3 + CH 3 → C 2 H 6 Formation of ethane

Evidence for this Free Radical Mechanism: Reaction requires UV light of energy high enough to homolyse chlorine to initiate. For every photon absorbed very many molecules of a product are formed. Therefore when a mixture of an alkane and chlorine are irradiated wIth UV light for even a short period, a chain reaction occurs – RXN STOPS IN THE DARK Formation of products such as ethane, chloroethane, butane etc. is evidence for the termination steps.

If radical promoters such as tetramethyl lead, Pb(CH 3 ) 4, or tetraethyl lead are added to the reaction mixture, there is a marked increase in the rate of the reaction. Pb(CH 3 ) 4 → Pb + 4CH 3

Syllabus requires that you know the mechanism of free radical substitution for both methane and ethane. Free Radical Substitution of Ethane: Propagation 1: Radical formed will be ethyl instead of methyl: C 2 H 6 + Cl → HCl + C 2 H 5 Tetraethyl lead, Pb(C 2 H 5 ) 4, is needed to increase the rate of the reaction

C 2 H 5 + Cl→ C 2 H 5 Cl Formation of chloroethane C 2 H 5 + C 2 H 5 → C 4 H 10 Formation of butane Termination stage – butane formed instead of ethane and some other possible combinations (Chlorine & Ethane): Cl + Cl → Cl 2 Formation of chlorine molecule

*Points to Note: - almost all organic compounds burn to form carbon dioxide and water - exceptions to this rule are the fully halogenated alkanes, CBrClF 2 - these type of compounds do not support combustion and are denser than air, hence, they are added to fabrics to reduce their tendency to catch fire. - known as FLAME RETARDANTS

Esterfication ( H atom of carboxylic acid replaced by alkyl group): Esterification (ester formation) is an example of a substitution reaction, with the carboxyl hydrogen in an acid replaced by the alkyl group of an alcohol This is also know as a CONDENSATION REACTION Definition: A condensation reaction is one in which two small molecules react to form a larger one with the elimination of a smaller one like water.

Ethanoic acid + Ethanol Ethyl Ethanoate + Water C H H H C O O-H C H H H-O C H H H + C H H H C O C H H O C H H H H O H + H+H+ H+H+ ESTERFICSATION HYDROLYSIS

Formation of an ester is also a reversible reaction i.e. a rxn in which the products react to form back the product and vice versa. In this case the reverse reaction is called HYDROLYSIS Definition: Hydrolysis is the chemical decomposition of a substance by water, the water itself also being decomposed.

Base Hydrolysis of Esters (Na/K replaces alkyl group): Esters can also be hydrolysed very effectively in the presence of a base like sodium hydroxide or potassium hydroxide. The base hydrolysis of esters using sodium hydroxide forms the sodium salt of the carboxylic acid (SOAP) rather than the carboxylic acid itself and alcohol. It is called BASE HYDROLYSIS OF AN ESTER or SAPONIFICATION reaction

Glyceryl + 3NaOH 3Sodium + Glycerol Tristearate Stearate (Soap) Ethyl + Sodium Sodium + Ethanol Ethanoate Hydroxide Ethanoate Sodium salt of carboxylic acid Basic Saponification of an Ester: *Important Saponification of an Ester:

*Syllabus requires that students are able to draw the structures of the reactants and products for soap manufacture!!!

C 17 H 35 C O O C H H H C H C H C O O C O O Triester – Glyceryl Tristearate (fat) + NaOH Sodium Hydroxide

C 17 H 35 C O O C O O C O O Na C H H H C H C H H-O + Sodium Stearate (Soap) Glycerol

Elimination Reactions

Definition: An elimination reaction is one in which a small molecule is removed from a larger molecule to leave a double bond in the larger molecule. Alkenes can be formed from their corresponding alcohols using elimination reactions. Since water is removed this particular type of elimination reaction is known as a DEHYDRATION REACTION The change in structure is from tetrahedral carbon (alcohol) to planar carbon (alkene)

Elimination Reaction – Preparation of Ethene Gas!!! Ethanol→Ethene + Water Saturated Unsaturated -H 2 O

Elimination Reaction – Preparation of Ethene Gas!!!! C H H H C H OH H Ethanol is a primary alcohol The functional group of an alcohol is O-H the hydroxyl group. A water molecule is eliminated C H H H C H OH H

C H H C H H H O H A double bond is formed between the carbons Ethene an unsaturated compound is formed +

The laboratory preparation of ethene involves elimination: X=Ethanol. Y=Hot aluminum oxide (Catalyst)

*Note: the removal of water from an alcohol to form an alkene is the only type of elimination reaction on the course!!! Question: Name two features of elimination reactions: Answer: - remove small molecule - make a double bond

ADDITION REACTIONS

Alkenes are much more reactive than alkanes and their characteristic reactions are ADDITION REACTIONS where the double bond opens up allowing various substances to add on and produce saturated compounds

C = C + Br-Br C-C H H H H H H H H Br The addition of bromine to a sample of ethene causes bromine to add across the C=C double bond to form 1,2-dibromoethane. This is an example of an addition reaction

If double bonds are stronger than single bonds why do alkenes react so readily with bromine??? Double bond consists of a sigma and a pi bond. Pi bonds are weaker than sigma bonds (less overlapping of orbitals) When bromine adds across the double bond in ethene the energy required to break the pi bond is released when two single bonds to the bromine atoms are formed. Products are more stable than reactants

Definition: An addition reaction is one in which two substances react together to form a single substance In general, an addition reaction involves a change in structure from planar to tetrahedral

Addition Reactions involving Ethene (Must be aware of the industrial importance of the addition reactions of ethene) a) Addition of Hydrogen (H 2 ): The addition of hydrogen to alkenes is known as HYDROGENATION Sunflower oil, palm oil etc. are said to be polyunsaturated (C=C) and are thought to be less damaging to our health than the saturated fats found in dairy products. Adding hydrogen to some of the C=C bonds in these oils changes them into soft solids. Thus hydrogenation is used in industry to convert vegetable oils into solid saturated materials used in margarine and dairy spreads

By controlling the degree of hydrogenation, the margarine can be made as hard or as soft as needed * Animal fats – saturated * Vegetable Fats unsaturated H H HH HH HH CC H H Ethene Ethane Catalyst Heat CC + H 2

B) Addition of Chlorine (Cl 2 ): The reaction between chlorine and ethene results in the product, 1,2-dichloroethane 1,2-dichloroethane, is used in industry to make chloroethene, the raw material for the manufacture of the plastic PVC - polyvinylchloride H H HH CC + Cl 2 Cl HH CC H H 1,2-dichloroethane

C) Addition of Bromine (Br 2 ): This reaction is used to test for unsaturation (preparation of ethene) Used as an additive in leaded petrol H H HH CC + Br 2 Br HH CC H H 1,2-dibromoethane

D) Addition of water (H 2 O): Addition of water is known as a HYDRATION reaction Reaction used in the manufacture of ethanol (a widely used solvent in industry) H H HH CC + HOH OHH HH CC H H Ethanol

E) Addition of Hydrogen Chloride (HCl): Main modern use of chloroethane is the manufacture of ethylcellulose, a thickening agent and binder in paints and cosmetics. H H HH CC + HCl ClH HH CC H H Chloroethane

In each of the reactions (a) – (e) the structure changes from planar to tetrahedral

Ionic Addition – Reaction Mechanism (Higher Level Only) C = C H HH H The double bond is made up of 2 bond pairs. 4 electrons in total. One pi bond and one sigma bond. It is an electron rich region with a slightly negative charge.

STAGE 1: Polarisation H H Br - Br.  A bromine molecule is a non- polar molecule However, as it approaches the double bond the high concentration of negative charge in the C=C bond causes the approaching Br-Br molecule to become polarised. C C H H

STAGE 2: Heterolytic Fission Br The induced polarisation becomes so great the Br 2 molecule splits into Br + and Br - species. This is known as heterolytic fission. *Heterolytic Fission: The breaking of a bond so that the bonding electrons(two) end up on one atom is known as heterolytic fission Br + + Br _

STAGE 3: Carbonium Ion Formation The Br + species in order to gain the electrons needed for a full outer shell attacks the C 2 H 4 molecule. The Br + ion forms a covalent bond with one of the carbon atoms. The other carbon atom has lost an electron and so becomes positively charged – CARBONIUM ION  H H C C H H Br + H H C C+C+ H H Br

*N.B. Note: As there is a shortage of electrons in this intermediate species, C 2 H 4 Br, it has an overall positive charge. Modern evidence shows that rather than bonding to one of the carbons, the Br + is attached to both in a bridged structure. This cyclic structure is known as cyclic bromonium ion The relatively large size of the bromine atom allows the formation of this three-membered ring structure C H HH H Br 

STAGE 4: Bromide ion attack on carbonium ion The presence of the carbonium ion makes the substance unstable and it quickly combines with the bromide ion, Br -, to form 1,2-dibromoethane C C+C+ H Br HH H Br - C C H Br H H H 1,2-dibromoethane

Evidence for Ionic Addition When ethene reacts with bromine in water in the presence of sodium chloride a number of compounds are formed: - 1,2-dibromoethane - 1-bromo-2-chloroethane – formed when the carbonium ion is attacked by the Cl - ion.

- 2-bromoethanol – formed when the carbonium ion is attacked by the water molecule

The syllabus requires you also know the mechanism of ionic addition of Cl 2 and HCl to ethene(Bk. Pg 369) *Note: a cyclic intermediate (stage 3) is not formed in the case of addition of Cl 2 or HCl to ethene. This is because the Cl atom and the H atom are to small to form a ring compound.