Halogenoalkanes Flashcards
Primary Secondary and tertiary Halogenoalkanes
Nucleophile
A nucleophile is an electron pair donor. Nucleophiles are e- rich species that are attracted to C delta + areas
Define Nucleophilic substitution and draw a general mechanism
A lone pair on the nucleophile is donated and used to form a covalent bond between the nucleophiles and the C delta +. The C - X bond breaks hetrolytically forming X- ion ie the X atom is swapped for the nucleophile
Nucleophilic substitution - OH- ions
Conditions:
The halogenoalkane must be warmed in an aqueous solution of either NaOH or KOH.
Nucleophilic substitution - CN- ions
Conditions:
NaCN or KCN must be dissolved in ethanol and heated under reflux.
Significance of Nucleophilic substitution with CN- ions in organic synthesis
It increases the C chain length by 1 carbon atom.
How to convert a CN- group in a nitrile into a carboxylic acid.
This involves reacting the Nitrile with HCl and H2O forming a carboxilic acid and NH4Cl.
This reaction is called hydrolysis.
Hydrolysis definition
A reaction in which a bond is broken by reacting it with H2O molecule.
Nucleophilic substitution - reaction with excess ammonia - How does ammonia behave as a nucleophile
In NH3, there is also a delta - charge on the N atom, but there is also a lone pair of electrons which are attracted to the C delta +. These e- and be donated and bond with the C to make a C-N bond
Nucleophilic substitution - reaction with excess ammonia - mechanism & equation & condition
Conditions:
Reaction needs to be heated inside a sealed copper tube.
The product formed is also a Nucleophile because there is a delta - charge on the N and a lone pair of e- on the N in the NH2.
Nucleophilic substitution - reaction with excess ammonia - How to discourage further substitution of this amine product and halogenoalkanes?
By using an excess of NH3 ????????????????? - what does this mean
How to compare the reactivity of halogenoalkanes to Nucleophiles
The reactivity of different halogenolkanes can be compared by reacting water with chloro-, bromo- and iodoalkanes. Although the reaction with water can be very slow, for testing purposes NaOH can be added to speed up the release of the Halide lon.
Eqn of Nucleophilic substitution of RX with H2O (where R represents the main chain and X is the halogen):
RX + H2O -> ROH + HX
This reaction can be monitored using a silver Nitrate solution (Acidified with HNO3):
- Add AgNO3 solution to the halogenoalkane . The water/NaOH reacts with the RX to give an alcohol and Halide ion X-. The X- ion produced will react with the Ag+ ions to form an insoluble ppt of AgX.
Describe an experiment you would do to compare the reactivity of the following halogenoalkanes CH3CH2I, CH3CH2Br, CH3CH2Cl to the nucleophile water:
- Put the same vol of each halogenoalkane in a test tube.
Add ethanol to the the mixture to help dissolve the halogenoalkane.
-To each, add the Same volume and concentration of AgNO3 solution to each (Which will contan H2O nucleophile). - Time how by each one takes to fom a ppt of Agx (have an X on a piece a paper and time how long it takes for it to disappear from sight).
Predict the reactivity trend of the halogenoalkanes
RI > RBr > RCl
The C-I bond requires the least energy to break (lowest difference in electronegativity) therefore the activation energy of the reaction will be the lowest and the reaction rate will be fastest. This is followed by RBr and then RCl
Elimination Reaction of Halogenoalkanes
Conditions:
NaOH or KOH dissolved in ethanol, sometimes known as ethanol or alchoholic NaOH.
Halogenoalkane + OH- -> Alkene + water + Halide ion
Ways Halogenoalkanes can react with NaOH / KOH
1) Nucleophilic substitution:
in this the OH - ion behaves as a nucleophile as the oxygen atom is donating a pair of electrons to carbon to form a new covalent bond.
2) Elimination
in this the OH - ion behaves as a base (a base is a substance which accepts protons)
Factors encouraging Nucleophilic Substitution by OH-
Solvent: water
Temperature: lower
Type of Halogenoalkane: 1 > 2 > 3
Factors encouraging an elimination reaction
Solvent: ethanol
Temperature: higher
Type of Halogenoalkane: 3 > 2 > 1
ChloroFluoroCarbons (CFCs)
CFCs are compounds in which all the hydrogen atoms have been replaced by Chlorine and/or Fluorine atoms. Given the strength of the C-F and C—Cl bonds these compounds are unreactive.
Due to their relative inertness and volatility they have been used extensively as aerosol propellants and refrigerants.
Ozone
Ozone is an allotrope of oxygen (O3). It is a pale blue gas with a sharp smell. At ground level it can cause pollution effects but high up in the stratosphere it is very useful - Ozone absorbs harmful uv light that can cause skin cancer
Formation of ozone from free radicals
Effect of CFC’s on the ozone layer
CFC’s can release Cl. Free radicals which react with ozone breaking it down into oxygen which depletes the ozone layer.
Benefit of HFC’s over CFC’s
