Halogenoalkanes Notes PDF

Learning Outcome 14: I can show an understanding of the properties, preparation and reactions of a number of organic compounds. TOPIC SUB-TOPIC ASSESSMENT CRITERIA 14.5 – Halogenoalkanes and 14.5.1 – Halogenoalkanes 1. I...

Learning Outcome 14: I can show an understanding of the properties, preparation and reactions of a number of organic compounds. TOPIC SUB-TOPIC ASSESSMENT CRITERIA 14.5 – Halogenoalkanes and 14.5.1 – Halogenoalkanes 1. Identify a given halogenoalkane group as primary, secondary or halogenoarenes classification tertiary. 14.5.2 – Halogenoalkanes 2. Describe the preparation of monohalogenoalkanes from alcohols and preparation alkenes. Limited to chloro-, bromo- and iodoalkanes. 3. Describe the preparation dihalogenoalkanes from alkenes and alkynes. Limited to chloro-, bromo- and iodoalkanes. 14.5.3 – Halogenoalkanes 4. Describe the conversion of halogenoalkanes: reactions i. by substitution into alcohols (by aqueous OH-), ethers, amines, nitriles, esters and alkylarenes; ii. by elimination into alkenes and alkynes (using alcoholic OH-); iii. into Grignard reagents; limited to monohalogenoalkanes; For (iii), description should include that dry conditions are required to prevent hydrolysis of Grignard reagents. iv. into alkanes by reaction with sodium under dry conditions. 106 Learning Outcome 14: I can show an understanding of the properties, preparation and reactions of a number of organic compounds. TOPIC SUB-TOPIC ASSESSMENT CRITERIA 14.5 – Halogenoalkanes and 14.5.4 – Halogenoalkanes 5. Explain the reactivity of monohalogenoalkanes in terms of their halogenoarenes reactivity structure (primary, secondary, tertiary halogenoalkanes) and the halogen atom (Cl, Br, I). 6. Explain the lack of reactivity of fluoroalkanes in terms of the C-F bond strength. TOPIC SUB-TOPIC ASSESSMENT CRITERIA 14.11 – Mechanistic aspects 14.11.1 – Ionic mechanisms 1. Describe the following mechanistic terms: nucleophile. 3. Describe bimolecular nucleophilic substitution (SN2) and unimolecular nucleophilic substitution (SN1) reactions of halogenoalkanes. 4. Explain why primary halogenoalkanes react by a bimolecular mechanism while tertiary halogenoalkanes react by a unimolecular mechanism. 107 HALOGENOALKANES - PREPARATION Halogenoalkanes (alkyl halides) contain a halogen atom, -X, 2. FROM ALKYNES bonded to an alkyl chain. ELECTROPHILIC ADDITION REACTIONS 1. FROM ALKENES a) Reaction with Cl2 or Br2 – forms tetrahalogenoalkanes. ELECTROPHILIC ADDITION REACTIONS a) Reaction with Cl2 or Br2 in inert solvent - forms vicinal dihalogenoalkanes. b) Reaction with HX – forms geminal dihalogenoalkanes Markovnikov’s rule applies. RECALL: Chloroethene is used as a feedstock for PVC manufacture. b) Reaction with HX – forms monohalogenoalkanes. Markovnikov’s rule applies. 108 2. FROM ALCOHOLS (Reactions involving R-O breaking) Halogenation (chlorination, bromination, iodination) Reaction with HCl, HBr and HI – forms monohalogenoalkanes. Preparation of hydrogen halides: Conc. aqueous HCl requires ZnCl2 catalyst (Lucas Test) HBr can be made in situ from KBr/H2SO4 HI can be made in situ from KI/H3PO4 Reaction with PCl3, PCl5 and SOCl2 (sulfur oxide dichloride) – forms chloroalkanes. PCl5 and SOCl2 give off HCl – used as a test for the –OH group. Reaction with PBr3 and PI3 – forms bromo- and iodoalkanes respectively. Preparation of PBr3 and PI3 PX3 can be formed in situ from red P and X2. 109 HALOGENOALKANES THE EFFECT OF THE LEAVING GROUP ON REACTIVITY Halogenoalkanes (alkyl halides) can be primary, secondary or In all nucleophilic substitution reactions, the carbon-halogen (C-X) bond tertiary. has to be broken at some point during the reaction. The harder it is to break, the slower the reaction will be. The C-F bond is very strong (stronger than C-H) and isn't easily broken. It doesn't matter that the C-F bond has the greatest polarity - the strength of the bond is much more important in determining its reactivity. It might therefore be expected that fluoroalkanes are very unreactive - and they are! RCH2X R2CHX R3CX In the other halogenoalkanes, the bonds get weaker - from chlorine to (primary RX) (secondary RX) (tertiary RX) bromine to iodine. This means that chloroalkanes react most slowly, bromoalkanes react faster, and iodoalkanes react faster still. The C-X bond readily undergoes nucleophilic substitution in Rates of reaction: RCl < RBr < RI halogenoalkanes. THE EFFECT OF THE HALOGENOALKANE STRUCTURE ON Nucleophilic substitution refers to any reaction in which an electron-rich nucleophile replaces a leaving group. REACTIVITY The polar bond in C𝛿+-F𝛿-, C𝛿+-Cl 𝛿- and C𝛿+-Br 𝛿- means that the carbon atom has a partial positive charge (𝛿+), which attracts Nucleophilic substitution proceeds via different mechanisms, depending substances with a lone pair of electrons such as :NH3, :CN- and :OH-. on whether the haloalkane is primary, secondary or tertiary. Tertiary halogenoalkanes react via a unimolecular nucleophilic Nucleophile: This is a species which donates an electron pair to a substitution (SN1) mechanism that has a much lower activation positively charged centre and forms a covalent bond with it. energy than the bimolecular nucleophilic substitution (SN2) mechanism of primary halogenoalkanes with a high energy transition state. Hence, tertiary haloalkanes react faster then secondary, which in turn react faster than primary. 110 HALOGENOALKANES SN1 mechanism SN2 mechanism A tertiary halogenoalkane forms a tertiary carbocation A primary halogenoalkane does not have its charge density intermediate. reduced as effectively as in a tertiary halogenoalkane, since there is only one alkyl group attached to the positive C atom. So the primary This is stabilised by the positive inductive effect of three alkyl groups carbocation would be a high energy transition state. (i.e. the three electron-pushing alkyl groups). This tends to reduce the charge density of the tertiary carbocation, lowering its energy. Mechanism Mechanism 1. The nucleophile, in a slow, rate-determining step, approaches 1. In an unmediated event, the halogen ion, X-, leaves the tertiary the haloalkane from the opposite side to where the halogen haloalkane in a slow reaction step. (The reason for this atom is (because of repulsions). As the nucleophile approaches happening is that the three electron-pushing alkyl groups the electron deficient carbon, the C-X bond lengthens. stabilize the carbocation that results.) 2. A very unstable, transition state, structure is reached where 2. Once formed, this highly reactive carbocation intermediate is the leaving halogen X and the incoming readily attacked by any nucleophilic species in the surrounding hydroxide nucleophile are equidistant from the carbon centre – both bonds are longer than normal. As soon as this stage is environment. reached, in a fast process, the halogen bond breaks. Since the nucleophile can approach the carbocation from two 3. A normal C-Nuc is now formed and the X- is released. [Only one opposite sides, a mixture of (potentially) optically active isomer forms in this mechanism]. isomers is possible. 111 HALOGENOALKANES … continued SN1 mechanism SN2 mechanism The formation of an intermediate with lower energy in tertiary halogenoalkanes (compared to the high energy transition state in primary halogenoalkanes), gives rise to the higher rate of reaction of tertiary halogenoalkanes. 112 HALOGENOALKANES – CHEMICAL PROPERTIES 1. NUCLEOPHILIC SUBSTITUTION REACTIONS d) NH3 – heating in a sealed container - forms a mixture of substituted ammonium salts (salts of amines). Excess NH3 forms primary amine a) Aqueous OH- (usually dilute, aqueous NaOH) - while heating under reflux – forms alcohols. Secondary and tertiary amines form if more halogenoalkane is present. Excess halogenoalkane forms quaternary ammonium salt, R4N+X-. b) CN- - refluxed in aqueous ethanol - form nitriles. c) Alkoxides (strong base) – at low temperatures - forms ethers. The alkoxide is usually formed by reacting KOH with alcohol. e) Carboxylates - usually while heating under reflux – form esters. 1o RX (especially smaller 1o RX (1-2 C) and with the smaller alkoxides (1-2 C)) are better at giving substitution reactions than either the 2o or (even less so) the 3o. The latter two also give elimination reactions in good yields. 113 HALOGENOALKANES – CHEMICAL PROPERTIES 2. ELIMINATION REACTION 3. OTHER REACTIONS Elimination of HX (not HF) forms an alkene if the halogenoalkane has a a) Wurtz reaction – two moles of halogenoalkanes react with 𝛽-H. sodium in anhydrous ether to form one mole of an alkane with twice the number of carbons. Elimination is brought about by the reaction of halogenoalkanes with an alkoxide (a strong base). The alkoxide is usually formed by reacting KOH with alcohol. Elimination is favoured by alcohol as a solvent, not H2O, and a higher temperature. b) Zn/Cu couple in 95% ethanol – forms an alkane with the same number of carbons c) Reaction of halogenoalkanes with Mg in ether solution and in anhydrous conditions forms Grignard reagents, RMgX. Note that dry conditions are required to prevent hydrolysis of the Grignard reagents: Substitution reaction (to form ether) competes. In fact, secondary and tertiary haloalkanes give elimination reactions in good yields, however, for the smaller primary haloalkanes (1-2 carbons) and with the smaller alkoxides (1-2 carbons), substitution reactions are preferred. 114  HALIDES – IDENTIFICATION DIHALOGENOALKANES TESTING FOR HALIDES A geminal or vicinal dihaloalkane on double Addition of silver nitrate to the resulting mixture shows up the dehydrohalogenation, in presence of a strong base (such as presence of halogen ions by way of precipitates: concentrated alcoholic KOH solution or fused KOH at 200 oC), results in If Cl- is present a white precipitate of AgCl, soluble in dilute the formation of an alkyne. aqueous ammonia, is obtained. If Br - is found in the mixture a pale yellow precipitate of AgBr, soluble in excess concentrated aqueous ammonia, is seen. If I- is present in the mixture a yellow precipitate of AgI, insoluble in ammonia, will form. 115 HALOGENOALKANES - USES USES OF ORGANOHALOGEN COMPOUNDS CFCs and the depletion of the ozone layer SYNTHETIC USE as per the previous slides Ozone was discovered in 1840. It was first detected in the upper layer of the atmosphere, the stratosphere in 1889. It is produced by the photochemical reaction between oxygen molecules and oxygen atoms USE AS SOLVENTS - Halogenoalkanes with more than one halogen in the atmosphere. atom dissolve oil and grease; they are used in the dry-cleaning O (g) + O2 (g) → O3 (g) industry and also for cleaning articles which carry a film of oil or grease from the machinery used in their manufacture. When ozone is formed at low altitudes, it can cause severe pollution These solvents are being phased out due to their toxicity and other problems. However, in the stratosphere, ozone becomes very important environmental impacts. to us. Ozone acts as a kind of shield, filtering out some of the harmful Examples: CCl4, CH2Cl2 – solvents in dry-cleaning ultraviolet radiation from the sun. If this radiation reaches the surface of the earth, there could be a drastic increase in the number of cases of skin cancer, genetic mutations and eye damage (e.g. cataracts being CFCs such as CFCl3, CCl2F2 and C2Cl2F4 AND OTHER HALOGENO formed). Ultraviolet radiation may also be harmful to marine life. COMPOUNDS Since 1976, there has been an alarming decrease in the amount of CFCs are compounds containing the elements C, F and Cl. ozone in the stratosphere over the South Pole. In recent years, a similar CFCs escape into the atmosphere and because of their lack of phenomenon has been occurring over the North Pole. reactivity, and their insolubility in water, there is no natural process for removing them. In fact, they drift up into the stratosphere (the upper atmosphere) where the presence of the high energy UV radiation initiates a chain reaction. … continued on the next slide 116 HALOGENOALKANES - USES … continued Scientists have discovered that the depletion of the ozone layer is caused by chlorofluorocarbons, commonly called CFCs. Over the years, CFCs have slowly diffused through the air and reacted with ozone (diagram adjacent), destroying the ozone layer. CFC compounds absorb UV radiation, which results in photodissociation reactions. For example: CF2Cl2 (g) + UV ⟶ Cl・(g) + CClF2・(g) CFCl3 (g) + UV ⟶ Cl・(g) + CFCl2・(g) Once released from their parent compounds, the highly reactive Cl・(g) can destroy O3 (g) molecules through a variety of catalytic cycles, the simplest being a reaction with O3 (g) to form an intermediate compound, chlorine monoxide (ClO・(g)), and finally … way forward O2 (g): Many countries have agreed to ban the use of CFCs. Cl・(g) + O3 (g) ⟶ ClO・(g) + O2 (g) In 1992, an international agreement was reached for a complete ban on the release of CFCs by 1996. Until now, most of the countries in the ClO・(g) + O (g) ⟶ Cl・(g) + O2 (g) world have completely banned the use of CFCs. However, even if the use of CFCs is totally stopped at once, the depletion of the ozone layer Overall reaction: O3 (g) + O (g) ⟶ 2O2 (g) will continue for many years due to the CFCs already present in the atmosphere. 117