Haloalkanes Flashcards
nucleophile
a species that can donate a lone pair of electrons to an electron-defiicnent carbon atom to form a dative covalent bond.
substitution, as applied to nucleophilic substituition in a haloalkane
carbon-halogen bond breaks and a bond is formed by the electron needing carbon atom with the nucleophile
hydrolysis
splitting up of a molecule using water.
bond enthalpy , as applied to a carbon-halogen bond
the amount of energy needed to split the carbon-halogen bond.
reaction concerning making a double covalent bond
elimination
reaction concerning replacing an atom
nucleophilic substitution
characteristic of the haloalkane molecule that enables it to undergo nucleophilic substitution
c-br bond is polar
reaction conditions: elimination
base-strong (high conc of NaOH), solvent is ethanol, temperature is high
reaction conditions:nucleophilic substitution
base-weak, solvent is water, temperature is low.
(a) Chloromethane can be made by the reaction of chlorine with methane.i- Give one essential condition for this reactionii- Name the mechanism for this reactioniii- Further substitution can occur during this reaction. Identify the main organic product when a large excess of chlorine is used in this reaction.
When we put a mixture of an alkane and a halogen into bright sunlight, or shine a photoflood lamp onto the mixture, the alkane and the halogen will react to form a haloalkane. The ultraviolet component of the light starts the reaction.a-iiThe mechanism of this reaction is free-radical substitution.a-iiiWhen an Alkane reacts with a Halogen, the halogen must replace a Hydrogen atom in the alkane. Therefore, the replaced Hydrogen will combine with the halogen forming 2 new products. Each time, Hydrogen Chloride will be produced.There are 4 stages in the reaction of Methane and Chlorine because the compound four hydrogen atoms.Stage 1: CH4 + Cl2 —–> CH3Cl + HCl (Chloromethane and Hydrogen Chloride)Stage 2: CH3Cl + Cl2 —–> CH2Cl2 + HCl (Dichloromethane & Hydrogen Chloride)Stage 3: CH2Cl2 + Cl2 —–> CHCl3 + HCl (Trichloromethane & Hydrogen Chloride)STage 4: CHCl3 + Cl2 —–> CCl4 + HCl (Tetrachloromethane & Hydrogen Chloride)The main organic product when a large excess of chlorine is used in this reaction is CCl4
(b) Ethanenitrile can be made by reacting chloromethane with potassium cyanide.ii- Name the mechanism for this reaction.iii- Explain, in terms of bond enthalpies, why bromomethane reacts faster than chloromethane with potassium cyanide.
b-iiThe mechanism of this reaction is a nucleophilic substitution.b-iiiC-X bond enthalpies are listed in the table below. The bonds get weaker as we go down in the group. Fluorine is the smallest atom of the halogens and the shared electrons in the C-F bond are strongly attracted to the fluorine nucleus. This makes for a strong bond. As we go down the group, the shared electrons in the C-X bond get further and further away from the halogen nucleus, so the bond becomes weaker.The bond enthalpies would predict the iodo compounds, with the weakest bonds, are the most reactive, and fluoro-compounds, with the strongest bonds, are the least reactive.In fact, experiments show us that reactivity increases as we go down the group. This means that bond enthalpy is a more important factor than bond polarity.
1-(a) State the benefit to humans of the presence of the ozone layer(b) Chlorofluoro compounds damage the ozone layer by the chlorine radicals reacting with ozone. Copy and complete the following equations to show how this reaction proceeds.Cl. +O3 —-> ? + ?? + O3 —–> Cl. + ?
Ozone is a molecule made from three oxygen atoms, O3 The ozone layer is important because it protects the earth from the harmful exposure to too many ultraviolet (UV) rays. Without this protective layer, life on earth would be very different. For example, plankton in the sea, which are at the very bottom of the food chain of the oceans, need protection from too much UV radiation. Also, too much UV radiation causes skin cancer in people by damaging DNA.Chlorine free radicals are formed from CFCs because C-Cl breaks homolytically in the presence of UV radiation to produce Chlorine free radicals Cl.Ozone molecules are attacked by these chlorine free radicals, Cl.Cl. +O3 —-> ClO. + O2 The resulting free radicals also attack ozone and regenerate Cl.ClO. + O3 —–> Cl. + 2O2
Give 3 uses of chlorofluorocarbons
refrigerantsair conditionerspropellants for aerosols
The reaction of bromine with ethane is similar to that of chlorine with ethane. Three steps in the bromination of ethane are shown below.Step 1 Br2 —–> 2Br.Step 2 Br. + CH3CH3 —–> CH3CH2. + HBrStep 3 CH3CH2. + Br2 —–> CH3CH2Br + Br.i- Name this type of mechanism.ii- Suggest an essential condition for this reaction.iii- Steps 2 and 3 are of the same type. Name this type of step.iv- In this mechanism, another type of step occurs in which free-radicals combine. Name this type of step. Write an equation to illustrate this step
iIt is called free radical substitution. It starts off a chain reaction which takes place in three stages: initiation, propagation and termination.iiWhen we put a mixture of an alkane and a halogen into bright sunlight, or shine a photoflood lamp onto the mixture, the alkane and the halogen will react to form a haloalkane. The ultraviolet component of the light starts the reaction.iiiPropagationThis take place in two steps:Step 2 Br. + CH3CH3 —–> CH3CH2. + HBrThe bromine free radical take a hydrogen atom from ethane to form hydrogen bromine, a stable compound. This leaves an ethyl free radical, CH3CH2.Step 3 CH3CH2. + Br2 —–> CH3CH2Br + Br.The ethyl free radical is also very reactive and reacts with a bromine molecule. This produces another bromine free radical and a molecule of bromoethane - a stable compound.ivTermination is the step in which the free radicals are removed. This can happen in any of the following three ways:Br. + Br. —–> Br2Two bromine free radicals react together to give bromine.CH3CH2. + CH3CH2. —–> C4H10Two ethyl free radicals react to give together butane.CH3CH2. + Br. —–> CH3CH2BrA bromine free radical and an ethyl free radical react together to give bromoethane
Further substitution in the reaction of bromine with ethane produces a mixture of liquid organic compounds.i- Name a technique which could be used to separate the different compounds in this mixture.ii- Write an equation for the reaction between bromine and ethane which produces hexabromoethane, C2Br6, by this substitution reaction
iFractional Distillationii6Br2 + C2H6 —–> 6HBr + C2Br6
