AQA AS Chemistry Halogenoalkanes PDF

AS CHEMISTRY 3.3.3 HALOGENOALKANES HALOGENOALKANES OVERVIEW DESCRIPTION General CnH2n+1X (where X is a halogen atom, F, Cl, Br, I) Formula Have...

AS CHEMISTRY 3.3.3 HALOGENOALKANES HALOGENOALKANES OVERVIEW DESCRIPTION General CnH2n+1X (where X is a halogen atom, F, Cl, Br, I) Formula Have had 1 or more H atoms substituted for a halogen Description atom Contain a polar bond C—X bond as halogens have high electronegativity. Polarity i.e δ+ δ- C—X Nucleophiles are attracted to the Cδ+ Reactions - Nucleophilic Substitution - Elimination Solubility in Insoluble. The polar bond is not polar enough to disrupt Water the hydrogen bonds between water molecules. Have higher melting and boiling points compared to their alkane equivalents as they have permanent dipole IMF’s between the molecules. Melting & Increase with increased chain length / Mr. Boiling Points Larger molecules have more electrons involved in the induced-dipole IMF’s making them stronger, so more energy is required to break them. AQA www.chemistrycoach.co.uk © scidekick ltd 2024 AS CHEMISTRY 3.3.3 HALOGENOALKANES NUCLEOPHILIC SUBSTITUTION Halogenoalkanes undergo nucleophilic substitution reactions. This is where a nucleophile substitutes in for the the halogen atom on the molecule. There are three versions of this mechanism that use different nucleophiles. The mechanism itself, however, is very similar for each. Nucleophile: OH- Reagent: NaOH(aq) Halogenoalkane to Alcohol H OH- H δ+ H C X H C OH + X- H H Overall equation: CH3X + NaOH CH3OH + NaX Nucleophile: CN- Reagent: KCN(aq) Halogenoalkane to Nitrile H CN- H δ+ H C X H C CN + X- H H Overall equation: CH3X + KCN CH3CN + KX AQA www.chemistrycoach.co.uk © scidekick ltd 2024 AS CHEMISTRY 3.3.3 HALOGENOALKANES Nucleophile: NH3 Reagent: Excess concentrated NH3(alc) + high pressure Halogenoalkane to Amine H NH3 H H H H δ+ H C X H C N+ H H C N H H H H H + X- + H+ Overall equation: CH3X + NH3 CH3NH2 + HX HINTS | TIPS | HACKS The mechanisms for producing an alcohol How to Draw Nucleophilic and a nitrile are exactly the same. Just a Substitution Mechanisms different nucleophile. Producing an amine has an extra step in the mechanism. It produces an intermediate, which then loses a H+ to leave the amine. Take note of the reagents and conditions for these. They could ask you in an exam. AQA www.chemistrycoach.co.uk © scidekick ltd 2024 AS CHEMISTRY 3.3.3 HALOGENOALKANES EASE OF SUBSTITUTION The “X” in each of the mechanisms could be F, Cl, Br, or I. The carbon to halogen covalent bond enthalpy is different for each of these. The C—F bond has the highest enthalpy, so is strongest. The C—I bond has the lowest bond enthalpy, so it is the weakest. Bond enthalpy (kJ.mol-1) C—F > C—Cl > C—Br > C—I strongest weakest The weaker the bond, the faster the rate of substitution. So if we were to compare the rates of substitution of different halogenoalkanes, we get: Rate of Substitution C—I > C—Br > C—Cl > C—F fastest slowest So, if ever you are asked which halogenoalkane has the fastest rate of substitution, the answer is the one with the halogen lowest down the group as it has the weakest C-X bond enthalpy, which requires less energy to break. AQA www.chemistrycoach.co.uk © scidekick ltd 2024 AS CHEMISTRY 3.3.3 HALOGENOALKANES ELIMINATION Halogenoalkanes can undergo an elimination reaction to produce an alkene. This process removes (eliminates) the halogen atom and a H atom from the molecule. Note that this only occurs for two carbon molecules and above. Otherwise an alkene (C=C) cannot form. Nucleophile: OH- Reagent: KOH(alc) (Note the alcoholic solution) Halogenoalkane to Alkene OH- H H H H H C C H C C + H2O + X- X H H H 1. The OH- attacks a H atom that is bonded to a C atoms next to the halogen group. 2. The bonding pair of electrons from the C-H bond go to fourth C=C and the H is released as a H+ (which neutralises the OH-) to form H2O. 3. The C=C causes the C-X bond to break and release the X-. In this reaction, the OH- acts as both a nucleophile and a base. AQA www.chemistrycoach.co.uk © scidekick ltd 2024 AS CHEMISTRY 3.3.3 HALOGENOALKANES ASYMMETRICAL HALOGENOALKANES Halogenoalkanes that are asymmetrical can form three different alkene isomers. e.g. 2-bromobutane When the OH attacks the H atom, the pair of electrons from the C-H bond can form a C=C on either side. or OH- H H H H H C C C C H H Br H H H H H H H H H H C C C C H H C C C C H H H H H H but-1-ene Z-but-2-ene and If it falls one way it forms but-1-ene. H H H If it falls the other it forms but-2-ene. However, but-2-ene has two H C C C C H geometrical isomers (E & Z). More on these in the Alkene section. H H H E-but-2-ene AQA www.chemistrycoach.co.uk © scidekick ltd 2024 AS CHEMISTRY 3.3.3 HALOGENOALKANES OZONE DEPLETION Ozone (O3) is formed naturally in the upper atmosphere of the Earth. 3O2 ⇌ 2O3 Ozone prevents harmful U.V. radiation rom the sun from passing through the atmosphere and harming life on the planet. However, CFC’s (chlorofluorocarbons), created by humans cause ozone to decompose in the upper atmosphere. CFC's were developed for use as solvents and refrigerants. 1. U.V. radiation causes the C-Cl bonds in CFC’s to break by homolytic fission, to form chlorine free radicals (Cl ) 2. Chlorine free radicals reacts with ozone to form a chlorate radical and oxygen. Cl + O3 ClO + O2 3. The chlorate free radical reacts with more ozone to produce oxygen and regenerate a chlorine free radical. ClO + O3 2O2 + Cl Make sure you know these equations! Scientific research provided enough evidence that this was occurring and, as a consequence, CFC’s were banned worldwide and, slowly, the ozone layer is regenerating. Chemists have since developed chlorine-free compounds for use as alternative solvents and refrigerants. Halogenoalkanes that do not contain chlorine are not a threat to the ozone layer as the C-X bonds in these dow more form free radicals in the presence of U.V. light. 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