Functional Group Analysis, Reactions & Mechanisms

9/25/2013FUNCTIONAL GROUP ANALYSIS, REACTIONS & MECHANISMS Keane Campbell MSc; BSc; ASc Created: November 12, 2012 Revised: September 19, 2013 Describe Selected Chemical Reactions of Alkanes • Alkanes are the least reactive class of organic compounds. They contain only C-C and C-H single bonds, which have high bond enthalpies. • Most useful reactions of alkanes take place under energetic or high temperature conditions. Alkane reactions often form mixtures of products that are difficult to separate. • The chemistry of alkanes is restricted to combustion and substitution reactions. Radicals are the only species reactive enough to overcome the high activation energy required to break the strong C-C and C-H bonds. 1 9/25/2013 Describe Selected Chemical Reactions of Alkanes Halogenation • Alkanes react with halogens (F2, Cl2, Br2, I2) to form alkyl halides via a mechanism known as free radical substitution. • Heat or light is usually needed to initiate this halogenation. The reaction of alkanes with chlorine or bromine proceeds at a moderate rate and is easily controlled. The reaction with fluorine is often too fast to control, while iodine reacts very slowly or not at all. • For example methane reacts with chlorine (Cl2) to form chloromethane, dichloromethane, trichloromethane and tetrachloromethane. . Describe Selected Chemical Reactions of Alkanes Cracking • This is used to convert higher-boiling fractions (larger hydrocarbons) into smaller ones. There are two types of cracking: a) Catalytic hydrocracking b) Catalytic cracking 2 A heterolytic cleavage is sometimes called an ionic cleavage. when a bond is broken and one of the atoms retains both electrons.9/25/2013 Describe Selected Chemical Reactions of Alkanes Combustion • This is a rapid oxidation that takes place at high temperatures.. . in such a way that each bonded atom retains one of the bond’s two electrons. • The bond-dissociation energy is the amount of energy required to break a particular bond homolytically. converting alkanes to carbon dioxide and water. we say that heterolytic cleavage has occurred. i.e. 3 . whereas heterolytic cleavage form ions. • Homolytic cleavage (radical cleavage) forms free radicals. there are certain important concepts that we have to become cognizant of. Explain the steps involved in the mechanism of free radical substitution • Students. prior to examining the mechanism of free radical substitution. • In contrast. Explain the steps involved in the mechanism of free radical substitution The Free Radical Chain Reaction • The free radical substitution is also known as the free radical chain reaction.9/25/2013 Explain the steps involved in the mechanism of free radical substitution • Note that a curved arrow ( ) is used to show the movement of the electron pair in an ionic cleavage. allowing the chain to continue until the supply of reactants is exhausted or the reactive intermediate is destroyed. separation. and to show the separation of individual electrons in a homolytic cleavage. what do we really mean when we speak of mechanism. which generates a reactive intermediate. • Remember. students. step-by-step description of exactly which bonds break and which bonds form in what order to give the observed products. • Now.e. side reactions that destroy intermediates and tend to slow or stop the reaction. • The mechanism is the complete. 2. in which the reactive intermediate reacts with a stable molecule to form another reactive intermediate. Propagation steps. • A chain reaction consists of three kinds of steps: 1. the curved arrow shows the movement of electrons. The initiation step.e. half arrows • The curved arrow begins with a covalent bond or unshared electron pair (a site of higher electron density) and points toward a site of electron deficiency. 4 . Termination steps. • Fish hook notation i. with all this said. while the fish hook shows the movement of one electron per atom i. reactive 3. we might expect the double bond to react and transform the pi bond into a sigma bond. Because single bonds (sigma bonds) are more stable than pi bonds. • The most common chemical reaction of a carbon-carbon double bond is the addition reaction. • Let us now look at the reactions of alkenes: 5 . The reactions of alkenes arise from the reactivity of the carbon-carbon double bond.9/25/2013 Describe Selected Reactions of Alkenes • All alkenes have a functional group: a carbon-carbon double bond. a) Bromination Alkenes react with bromine in the presence of an organic solvent (CCl4) to form 1. b) In the presence of water. thus it does not participate in the reaction. Br2 (l) Note the organic solvent is inert. molecules of the solvent become reactants.2-dibromoalkane. Explains the steps involved in the mechanism of selected chemical reactions of alkene functional group Electrophilic Addition of Bromine • Electrophiles are species that are electron-pair acceptors.9/25/2013 Describe Selected Reactions of Alkenes 1. Br2 (aq).e. too: Note that when bromine reacts with an alkene in the presence of water it is aqueous bromine i.e. Electrophiles accept electron pairs to form new bonds. They are typically positive ions like H+ or NO2+. a halo alcohol. there will be a partial positive and a partial negative charge on each of the bromine atom. causing it to look like this: The mechanism for the electrophilic addition of bromine to alkene occurs in two steps: • • 6 . Note that when bromine in the presence of an organic solvent it is liquid bromine i.e. The bromine is a very “polarisable” molecule and the approaching pi bond in a molecule i. halogens add to alkenes to form halohydrins i. In this case.e. warm concentrated KMnO4 oxidizes alkenes to glycols. H2SO4 at room temperature. To be even more specific. the purple solution becomes colourless. b) Alkenes react with cold KMnO4. Describe Selected Reactions of Alkenes 3. Concentrated Sulphuric Acid • When conc. but the version in the equation is better because it shows how all the atoms are linked: 7 . The manganate (VII) ions are reduced to manganese (II) ions while the alkenes are converted to 1. Oxidation a) Alkenes react with warm acidified potassium manganate (VII) solution to form mixtures of ketones and carboxylic acids. then cleaves the glycols. The products are initially ketones and aldehydes. but aldehydes are oxidized to carboxylic acids under these conditions.2-diols. H2SO4 adds across a carbon-carbon double bond an alkyl hydrogen sulphate is produced. When ethene is bubbled into conc. If the KMnO4 solution is acidified with dilute H2SO4. ethyl hydrogen sulphate is formed: • • The structure of the product molecule is sometimes written as CH3CH2H2SO4.9/25/2013 Describe Selected Reactions of Alkenes 2. students. The addition of hydrogen bromide to prepare could give either • In fact. formulated a rule for predicting which addition product would be formed. The Russian chemist. the product is almost entirely 2-bromopropane. HCl. • Describe Selected Reactions of Alkenes Electrophilic addition of hydrogen bromide A molecule of hydrogen halide is permanently polarised. all add across the double bond. Markovnikov’s Rule can be stated: in addition of a compound HX to an unsaturated carbon atom which carries the larger number of hydrogen atoms. HBr. bromoethane is formed: • When hydrogen halides add to an unsymmetrical alkene two products are possible. 8 .9/25/2013 Describe Selected Reactions of Alkenes 4. • Hydrogen halides The hydrogen halides. Markovnikov. and HI. When hydrogen bromide adds to ethene. 9 . eliminating a double bond and making them more saturated. However. which makes them attractive for baking and extends their shelf-life. Describe Selected Reactions of Alkenes 4. the process frequently has a side effect that turns some cis-isomers into transunsaturated fats instead of hydrogenating them completely. The process of hydrogenation is intended to add hydrogen atoms to cis-unsaturated fats. allowing other chemicals and enzymes to bind to them. • Hydrogenation This is the process by which hydrogen is added across a double bond at high temperature in the presence of finely nickel. one hydrogen atom adds to one side of the structure and the other atom to the other side. it creates trans fats. If both hydrogen atoms are added to the same side of the structure. it is not possible to control where the hydrogen atoms are added to the structure. • If.9/25/2013 Describe Selected Reactions of Alkenes 4. Hydrogenation • Cis fats exist naturally and. If both hydrogen atoms are added to the same side of the structure. • • • When this oil is hydrogenated. which is what makes it ‘unsaturated’ (saturated fats have no double bonds and all the ‘species’ available are taken up by hydrogen atoms. they bend. platinum or palladium catalyst. unsaturated fatty acid might look like the molecule below. however. A natural. These saturated fats have a higher melting point. because the hydrogen atoms are crowded on one side of the molecule. It has several double-bonds between adjacent carbon atoms. it is called a ‘cis’ fat. 4. Because the structure is uncrowded. which allows them to solidify at room temperature. Why do alkenes show geometrical isomerism. Hydrogenation • Trans fats do not exist naturally. Trans fats are straight and can pack into a crystal formation. [4 marks] a) b) Br2 in H2O Br2 in CCl4 3. Trans fats have harmful effects on blood lipids. and name the product. [4 marks] Write and name the structural formula for the products that form when ethene reacts with each of the following reagents. with a very few exceptions. and cause blood vessel abnormalities. whereas alkanes do not? [2 marks] Outline mechanism for the electrophilic addition of HBr to 2-methyl-1-butene. • • • Graded Worksheet Answer all the questions below: 1. 2. • The shape of the molecule is therefore vital to its function. [6 marks] CH3CH=CHCH2CH2CH3 CH3C(CH3)=CHCH2CH(CH3)2 CH3CH2CH=CH(CH2)2CH3 CH3CHClCHCH2 CH3CHCH(CH2)2CH3 (CH3)2C=C(CH3)2 10 . all of which are risk factors for heart disease. Draw and label the isomers of 3-hexene using both the cis-trans.9/25/2013 Describe Selected Reactions of Alkenes 4. they do not bend and so other molecules and enzymes find it more difficult to bind them. Trans fat raises your “bad” low-density lipoproteins (LDL) cholesterol and lowers your “good” (HDL) cholesterol. [6 marks] Name the following compounds: a) b) c) d) e) f) 5. promote inflammation. much in the same way as the shape of a key is important for operation of a lock. • in a tertiary alcohol. it has two R groups and one hydrogen atom. Nucleophiles are species that are electron-pair donors.e. • in a secondary alcohol.9/25/2013 Describe Selected Chemical Reactions of Alcohols • Alcohols contain the functional group –OH attached to an alkyl group. Nucleophiles include negatively charged ions such as CN-. • Now students. to what class would methanol belong? Describe Selected Chemical Reactions of Alcohols The most typical reactions of alcohols involve nucleophilic substitution reactions in which the –OH group is replaced by an incoming group such as a halogen. as well as molecules bearing lone pairs of electrons like H2O or NH3. 11 . • The carbon that the –OH group is attached to is referred to as the carbinol carbon atom.or Cl-. • Secondary (20) • Tertiary (30) • The classification is based on the number of alkyl groups (R) attached to the carbinol carbon atom i. it has one R group and two hydrogen atoms. • There are three main classes of alcohols: • Primary (10). it has three R groups. the carbon bearing the –OH group: • in a primary alcohol. Nucleophiles attack the partially positively charged (δ+) atoms of polar covalent bonds. OH. based on these classifications of alcohols. (aq) to the green colour of Cr3+ (aq). Tertiary alcohols are resistant to oxidation. 12 . Acidified K2Cr2O7 solution at room temperature will oxidize primary alcohols to aldehydes and secondary alcohols to ketones. ethanol and ethanoic acid react in the presence of a concentrated sulphuric acid and catalyst to form the ester ethyl ethanoate: In this reaction. This is too powerful an oxidizing agent to stop at the aldehyde: it oxidizes primary alcohols to carboxylic acids. For example. It oxidizes secondary alcohols to ketones. H+/KMnO4 solution. Describe Selected Chemical Reactions of Alcohols Esterification Alcohols and carboxylic acids react to give esters. A number of oxidizing agents can be used.9/25/2013 Describe Selected Chemical Reactions of Alcohols Oxidation Primary alcohols are oxidized to aldehydes and to carboxylic acids. The dichromate solution turns from the orange colour of Cr2O72. Secondary alcohols are oxidized to ketones. At higher temperatures primary alcohols are oxidized further to acids. A powerful acidic oxidizing agent converts them into a mixture of carboxylic acids. it is the C(O)–OH bond in the acid that breaks and not the C–OH bond in the alcohol. I will show the mechanism so as to enable better understanding of the aforementioned paragraphs. An equilibrium is established between reactants and products. followed by loss of a proton. According to the syllabus only equations are required for the reaction of alcohol with carboxylic acid and conc. Dehydration requires an acidic catalyst to protonate the hydroxyl group of the alcohol and convert it to a good leaving group. gives the alkene. Describe Selected Chemical Reactions of Alcohols Dehydration Reactions Now students. Loss of water.9/25/2013 Describe Selected Chemical Reactions of Alcohols Dehydration Reactions Dehydration is the removal of water. H2SO4. 13 . for teaching purposes only. . It is insoluble in water and has an antiseptic smell. A positive result is the appearance of a very pale yellow precipitate of triiodomethane (previously known as iodoform) –CH3. Iodoform (CHI3) is a pale yellow substance. NaOH) This is a positive test used to identify alcohols. followed by just enough sodium hydroxide solution to remove the colour of iodine. the carbon attached to the halide. Due to its high molar mass due to the three iodine atoms. a positive reaction gives iodoform.9/25/2013 Describe Selected Chemical Reactions of Alcohols Iodoform Test (I2. Iodine solution is added to a small amount of an alcohol. If nothing happens in the cold it may be necessary to warm the mixture very gently. It is used as an antiseptic on the sort of sticky plasters you put on minor cuts.e. • Tertiary (30) – if there are three alkyl groups attached to the α carbon. it is solid at room temperature. Describe Selected Reactions of Halogenoalkanes • Halogenoalkanes are classified depending on the number of alkyl groups attached to the carbon atom bonded to the halogen: • Primary (10) – if there is one alkyl group attached to the α carbon i. • Secondary (20) – if there are two alkyl groups attached to the α carbon. When iodine and sodium hydroxide are used. 14 . there is bond breaking and bond making taking place.is replaced by an –OH ion. • Step-wise manner is. • E.9/25/2013 Describe Selected Reactions of Halogenoalkanes • Hydroxide ions hydrolyze halogenoalkanes to alcohols. as it is suggested by its name. Describe Selected Reactions of Halogenoalkanes • Depending on the nature of the halogenoalkane. • The hydroxide ion is a strong nucleophile and the halide ion is a nucleophile. • Now let us look at the word concerted. Bromoethane will form ethanol wherein the Br.g. Whenever a halogenoalkane is hydrolyze the corresponding alcohol is formed.e. A concerted reaction is one in which both bond breaking and making are occurring at the same time. i. • The reason is simple students. bond breaking before bond forming. however. 15 . whenever we have a nucleophilic substitution for a halogenoalkanes. • For alkylhalide to have a concerted reaction it would have to be a primary alkylhalide. whether it be a primary or secondary alkyhalide (halogenoalkane). • Now students. one process occurring before the other. • Therefore we have one nucleophile replacing another nucleophile in the hydrolysis of halogenoalkanes which in essence is a nucleophilic substitution reaction. the bond breaking and making processes may occur in a • concerted manner or • Step-wise manner. a primary alkylhalide is unable to form a stable carbocation intermediate to await the entry of the (-OH) hydroxide ion. (In general. • Hydroxide ion attacks the backside of the electrophilic carbon atom. transition state is not a discrete molecule that can be isolated. so the carbon-iodine bond must begin to break as the carbon-oxygen bond begins to form. it only exists for an instant. Hydrolysis of a Primary Halogenoalkane • Carbon can accommodate only eight electrons in its valence shell. taking place in a single step with bonds breaking and making at the same time. from the electron-rich nucleophile to the carbon atom of the electrophile. rather than an intermediate. 16 . donating a pair of electrons to form a new bond. not the other way around. • Note students.9/25/2013 Hydrolysis of a Primary Halogenoalkane • The reaction of ethyl iodide (bromoethane) with hydroxide ion is a concerted reaction. a point of maximum energy. • In this transition state. nucleophiles are said to attack electrophiles.) • Notice that curved arrows are used to show the movement of electron pairs. • The middle structure is a transition state. the bond to the nucleophile (hydroxide) is partially formed and the bond to the leaving group (iodide) is partially broken. The second step is a fast attack on the carbocation by a nucleophile. 17 . The first step is a slow ionization to form a carbocation. Aldehydes have the structure (as given above) where R is an alkyl or aryl group. Describe Selected Chemical Reactions of Carbonyl Compounds • The carbonyl group consists of an oxygen atom attached to a carbon atom by a double covalent bond. usually represented by the formula • Aldehydes and ketones are two classes of compound that both contain the carbonyl group. Step 1: Formation of carbocation (rate-determining) Step 2: Nucleophilic attack on the carbocation • Note the long arrow is suggesting that reaction is more stable on the reactant side.9/25/2013 Hydrolysis of a Tertiary Halogenoalkane • The reaction of 2-bromo-2-methylpropane (t-butyl bromide) with hydroxide ion occurs in a step-wise manner forming an intermediate in the process. to a brickred precipitate of copper (I) oxide Cu2O. 1) Tollen’s reagent: the ‘silver mirror’ test Tollen’s reagents is sometimes called ‘ammonical silver nitrate. aldehydes reduce Fehling’s solution. which is blue. is reduced to a silver mirror when warmed with aldehydes but not ketones.4-dinitrophenylhydrazine in methanol and sulphuric acid is called Brady’s reagent. So formation of a yellow-orange precipitate of a DNP derivative indicates the presence of an aldehyde or ketone. 4 – DNP (Brady’s Reagent) Brady’s reagent is used to test for the presence of carbonyl compounds. Fehling’s solution Fehling’s solution contains complex copper (II) ions. If carbonyl compound is present: aliphatic carbonyl compounds give orange or yellow precipitate. There are two ways in which we can differentiate between an aldehyde and a ketone. A solution of 2. [Ag(NH3)2]2+. (Methanal reduces to solution further to metallic copper.) 2) 18 . It is a solution of complex silver ions. while those of aromatic carbonyl compounds are darker in colour. On warming. Distinguishing Between Aldehydes & Ketones Aldehydes have reducing properties whereas ketones do not have reducing properties.9/25/2013 2. pressure and a metal catalyst (Pt or Ni). So remember students: • Aldehydes are reduced to 10 alcohols by H2 in the presence of Pt or Ni and by LiAlH4 in ether. Cyanohydrin (Hydroxynitirile) Formation (NaCN/HCl) A cyanohydrin reaction is an organic chemical reaction by an aldehyde or ketone with a cyanide anion to form cyanohydrin.is a nucleophile which attacks the carbonyl group causing it to become polarize and picking the H+ of HCl.9/25/2013 Distinguishing Between Aldehydes & Ketones Oxidation [KMnO4 (aq)/H+(aq)] Aldehydes are readily oxidized to carboxylic acids by acidified potassium manganate (VII). in acid conditions. reducing aldehydes to primary alcohols and ketones to secondary alcohols. With a powerful oxidizing agent. The reaction is more difficult than addition to a carbon-carbon double bond. and usually requires heat. • Ketones are reduced by H2 in Pt or by LiAlH4 in ether to give 20 alcohols. they are oxidized to a mixture of carboxylic acids. Reduction of carbonyl compounds Aldehydes are reduced to primary alcohols by H2/(Pt or Ni) Hydrogen adds across the carbonyl group. The CN. Ketones are more difficult to oxidize than aldehydes. LiAlH4 LiAlH4 is used in solution in ethoxyethane to reduce aldehydes to primary alcohols and ketones to secondary alcohols. Distinguishing Between Aldehydes & Ketones Reduction of carbonyl compounds Aldehydes are reduced to primary alcohols by Lithium tetrahydridoaluminate. 19 . 9/25/2013 Mechanism of Selected Chemical Reactions of Carbonyl Compounds • Mechanism of Selected Chemical Reactions of Carbonyl Compounds 20 . carbonates.)2Mg2+ (aq)+ H2 (g) C3H7COOH (aq) + Mg (s) (C3H7COO-)2Mg2+ (aq) + H2 (g) The evolution of CO2 from sodium hydrogen carbonate is used as a test to distinguish carboxylic acids from weaker acids.Na+ (aq)+ H2O (l) HCOOH (aq) + NaOH (aq) HCOO-Na+ (aq) + H2O (l) RCOOH (aq) + NaHCO3 (aq) RCOO. PCl3 (phosphorus trichloride) and PCl5 (phosphorus pentachloride) to produce acyl chlorides. hydrogen carbonates and alkalis: RCOOH (aq) + NaOH (aq) RCOO. The conversion of carboxylic acids to acyl chlorides using thionyl chlorides (SOCl2) is more convenient than PCl3 and PCl5 since the by-products formed.9/25/2013 Selected Chemical Reaction of Carboxylic Acids Salt Formation Carboxylic acids form salts by reaction with metals. using thionyl chloride are gases. Conversion to Acid Chlorides Carboxylic acids react with SOCl2 (thionyl chloride). 21 . Selected Chemical Reaction of Carboxylic Acids Esterification Carboxylic acids react with alcohols in the presence of concentrated sulphuric acid to form esters: The alkyl group after the ester linkage is written first.Na+ (aq) + H2O (l) + CO2 (g) CH3COOH (aq) + NaHCO3 (aq) CH3COO-Na+ (aq) + H2O (l) + CO2 (g) 2RCOOH (aq) + Mg (s) (RCOO. such as phenols. Example Transesterification: alcohol + ester  different alcohol + different ester Example 22 . literally means ‘the making of soap. Ester hydrolysis The hydrolysis of an ester gives an equilibrium mixture of carboxylic acid and alcohol. in acid hydrolysis of water you end up with your starting materials (what you use to make the esters).’ Transesterification: Biodiesel Production Transesterification is the process of exchanging the organic R’’ of an ester with the organic group R’ of an alcohol.9/25/2013 Selected Chemical Reaction of Esters Esters contain the functional group: where R and R1 may be alkyl groups and may be the same or different. The attachment of equilibrium is catalysed by the presence of mineral acids: CH3CO2C2H5 (l) + H2O (l) ⇋ CH3COOH (aq) + C2H5OH (aq) Simply put students. These reactions are often catalysed by the addition of an acid or base. called saponification. Basic Hydrolysis Basic hydrolysis of esters. as well as an acid or base are used to help the reaction proceed more quickly. The mixture of fatty acids is the biodiesel. students. Heat. Transesterification: Biodiesel Production A catalyst and a base is used in the process. transesterification is an example of hydrolysis of esters. Animal and plant fats and oils are typically made of triglycerides which are esters.9/25/2013 Transesterification: Biodiesel Production The name ‘biodiesel’ has been given to transesterified vegetable oil to describe its use as a diesel fuel. Normally this reaction will proceed either exceedingly slowly or not at all. One of the first uses of transesterified vegetable oil (biodiesel – vegetable oil is an ester) was to power heavy – duty vehicles in South Africa before World War II. Note. 23 . Ethanol and methanol are commonly used in transesterification process.