Halogenoalkanes Flashcards
The 3 main nucleophiles are
OH-
CN-
NH3
Define nucleophile, electrophile and curly arrow
•Nucleophile is an electron pair donor (must have lone pair)
•Electrophile is an electron pair acceptor
•Curly arrow shows the movement of a pair of electrons (must either start from a lone pair or covalent bond)
Reactions with aqueous hydroxides - hydrolysis (nucleophilic substitution)
Hydrolysis is the breaking of a bond using water
•Forms alcohols
•Reagent - NaOH(aq) / KOH(aq)
•Conditions - warm & aqueous
General equation:
R-X + MOH —> R-OH + MX
Mechanism contains:
•Lone pair and negative charge on OH where OH = nucleophile
•Partial charges
•Curly arrow from lone pair to C and curly arrow from covalent bond to X
Reactions with Ethanolic Potassium Cyanide (nucleophilic substitution)
•Forms nitriles
•Reagent - KCN
•Conditions - aqueous ethanol
General equation:
RX + KCN —> R-C≡N + KX
•This increases the length of the carbon chain
Mechanism contains:
•Lone pair and negative charge on CN where CN = nucleophile
•Partial charges
•Curly arrow from lone pair to C and curly arrow from covalent bond to X
•Show triple bond like this C≡N
Reaction with excess ammonia (nucleophilic substitution) - DIFF TO THE OTHER 2
•Forms amines (R-NH2)
•Reagent & Conditions - excess ammonia (NH4)
General equation:
R-X + 2NH3 —> R-NH2 + NH4X
•If NH2 at end of chain the IUPAC name would be smt like methylamine
•If NH2 not at end of chain the IUPAC name would be smt like 2-aminopropane
•If NH2 is on both ends, IUPAC name would be smt like propane-1,3-diamine
Mechanism contains:
•Lone pair but NO negative charge on NH3 where NH3 = neutral nucleophile
•Partial charges
•Curly arrow from lone pair to C and curly arrow from covalent bond to X
2nd part of mechanism contains:
•R-NH3 with the ammonia in displayed formula
•Positive charge on the N atom
•Curly arrow going from covalent bond of
N+ — H to N+
The rate of substitution reactions of halogenoalkanes depends on
The carbon-halogen bond enthalpy
•Highest bond enthalpy = most energy required to break
Test:
•1-Chloro-, 1-bromo- and 1-iodobutane are each warmed to 60 °C with aqueous AgNO3, using ethanol as a mutual solvent (dissolves both AgNO3 / H2O + RX)
•Rate of reaction measured by timing how long it takes for a precipitate of silver halide to form
To ensure the test is fair:
-Use equal amts (in mol) of each RX
-Use RXs with the same chain length
-Use water bath to ensure constant temp
Results of the test
•1-chlorobutane = white precipitate (slowest time)
•1-bromobutane = cream precipitate
•1-iodobutane = yellow precipitate (fastest time)
Explanation:
•C-I bonds are weaker than C-Cl or C-Br bonds so break more easily => C-I bonds react faster
This reaction cannot be used to determine the rate of hydrolysis of 1-fluorobutane as AgF is soluble so precipitate not formed.
Elimination reactions
Elimination is a reaction in which an atom or group of atoms is removed from a reactant to form a double bond.
•Reagent - NaOH / KOH
•Conditions - HOT , ethanol (as solvent)
•The halogen atom is always removed
•H+ ion from an adjacent C atom is also removed
•C=C forms
•Multiple isomeric alkenes can be formed
Mechanism contains:
•Lone pair and negative charge on OH where OH- = base
•3 curly arrows:
-from covalent bond of R-X to the X atom
-from lone pair to H atom on the adjacent carbon (NEXT TO THE CARBON THAT IS BONDED TO A HALOGEN)
-From the covalent bond of C-H going to the covalent bond of C-C
Difference between substitution and elimination
Conditions:
•Substitution - Warm and Aqueous
•Elimination - HOT and ethanol
Major product:
•Substitution - alcohol
•Elimination - alkene
Role of OH- :
•Substitution - nucleophile
•Elimination - base
CFCs and the environment
CF2Cl2 —> ĊF2Cl + Cl•
-Needs UV radiation
O3 + Cl• —> ClO• + O2
03 + ClO• —> Cl• + 2O2
2O3 —> 3O2
