Chemistry- Unit 2- Haloalkanes

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  • Created by: FireDwarf
  • Created on: 27-02-14 18:04
General formula for a haloalkane?
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Naming- Carbon atoms? Abriviations about the halogens? Counting? Alphabet?
(Methane,ethane,propane,butane.pentane,hexane), fluoro,chloro,bromo,iodo. Always get shortest chain & shortest distance to halogen. Alphabetise them correctly. Alphabet BEFORE number.
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Bond Polarlarity?
Convalent bond between C-X. Halogens are more electronegative then a carbon atom, therefore causing a slight negative charge on the halogen.
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How does electronegativity change down the group?
Become less strong because more shielding, therefore increased distance from P nucleus to bonded electrons, so less attraction. Therefore, less polarity.
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How does their solubility change?
Polar bond not strong enough to make haloalkane soluable. Halogalkane therefore mixes with hydrocarbons to be dry cleaning fluids.
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Boiling Point?
Increases with increasing chain lengh & as we move down the group. Both due to increased van der walls and therefore more intermolecular forces.
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Why does the convalent bond strengh of C-X increase up group?
F is a small atom and therefore shared electrons in the C-F bond are strongly attracted to the F nucleus.
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What is a nucleophile? (Citation needed)
Reagants that attack and form bonds with + charged carbon atoms.
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What can it be? What does it need?
Negativly charged ion or atom with slightly -charge. It needs a lone pair of electrons which must be situated on a very electronegative atom.
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Common nucleophiles? (3)
Hydroxide ion, ammonia, cyanide ion.
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What do they do in the reaction?
They substitue the halogen in the haloalkane (replace them)
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What is the very simple not reaction mechanisim equation?
Haloalkane + the nucleophile (which has a - charge + lone pair) --> alkane with the nucleophile on it (no longer - charged) + a negativly charged halogen with a lone pair of electrons.
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Curly arrow reaction mechanisim?
Draw the haloalkane. Draw the Nucleophile in the top right corner. Curly arrow from the lone pair towards the carbon. Second arrow from the bond between C-X moving electron pair to the halogen atom to produce the halide ion.
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What is special about the Ammonia?
Theres 2 moles, so therefore 2 nucleophiles. First one is the same, but in the second one, The Nh3 takes a hydrogen + to form Nh4X. One the second, first arrow goes from Ammonia to 1 Hydrogen atom and the second arrow goes from N to bond N-H.
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How does hydrolysis of haloalkanes work?
O in a water molecule has 2 lone pairs so acts as a nucelophile.
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How can the rate of the reaction work?
Because we are forming halides, we can use silver nitrate (the coloured tests).
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What causes substritution to occur instead of elimnation?
Diffrence in conditions.
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What happens during elimination?
A hydrogen atom (which is slightly positivly charged) forms a bond with a ion (eg OH-) . The bromine takes electrons from the C-Cr and becomes a bromide ion. Double bond formed between C=C (alkene).
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What is causing the elimination for instance using OH-?
The OH- is acting as a base. It therefore removes a H+ ion from the haloalkane
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How do we draw the reaction mechanisim for this?
3 arrows. First arrow from the base (OH-)'s electrons to the H. Second arrow from the H-C bond to the C-C bond. 3rd arrow from the C-Br bond to the Br.
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How do we know if its substritution or elimination?
Hydroxide ions at room temperture dissolved in water favour substrituion. While hyoxide ions at high tempertures dissolved in ethanol favour elimination.
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What else decides?
Whether it is primary (which favours substrituion), secondary (favours both), tertiary (favours elimination)
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What do those 3 words mean?
Primary- Halogen atom at the end of the chain, secondary- in the body of the chain, tertiary, on a branch of the chain.
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What is the name given to haloalkanes which contain both CL and F atoms?
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Some uses?
shorter chains, gases, so for aerosol propellants etc. Longer chains for dry cleaning and de-greasing solvents.
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What causes alkanes and halogens to form haloalkanes? Equation?
UV light must be present. alkane (CnH2n+2) + halogen --> Haloalkane l (CnH2n+1) + 1 atom of the halogen + a H g (eg: HBr)
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What is this reaction called? What are the 3 stages?
Free radical substritution. Initiation, propagation, termination.
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What occurs in initiation?
The halogens bond must be broken (eg Cl2 has Cl-Cl). Chrlorine molecule abosrbs energy of UV light. Energy greater then bond strengh, so it breaks.
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What does this form?
Two seperare chlorine atoms that are free radicals and have 1 dot on their right hand side. Cl-Cl ---> (UV) 2Cl(DOT).
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Why does the C-H bond not break?
It is stronger and therefore UV light quantum cannot provide sufficent energy to break it.
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What occurs in propagation?
halogen free radical reacts with alkane. It takes a H from the alkane to become stable, which leaves the alkane as unstable therefore. But this is reactive, so to become stable, reacts with a halogen molecule (Cl2) leaving a halogen free radical.
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What has happened?
The halogen free radical has regenerated itsself.
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What happens in termination?
Free radicals are removed. Can occur by two free radicals reacting to form a stable compound.
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Give an example through all the stages.
Ch4+ Cl2--> Ch3cl + Hcl. INITIATION --> Cl-Cl --> (UV) 2Cl(dot). PROPAGATION-->Cl(dot) +Ch4--> HCL + (dot)Ch3. (dot)CH3 + Cl2 --> Ch3cl + Cl(dot).
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TERMINATION --> Cl(dot)+Cl(dot) --> Cl2. (dot)CH3 + (dot)CH3--> C2H6. Cl(dot) +(dot)Ch3 --> CH3Cl.
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Why are chain reactions important?
CFCS destroy ozone layer. Chlorine free radicals form which then take an O from the O3 to form ClO(dot) + O2. Then regenerate by Clo(dot) + 03 --> 2O2 + Cl(dot).
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Naming- Carbon atoms? Abriviations about the halogens? Counting? Alphabet?


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How does electronegativity change down the group?


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