Halogenoalkanes Flashcards
How do you name a halogenoalkane?
- Prefixes flouro, iodo, bromo, chloro are used
- Numbers are used to tell you which carbon the halogen is bonded to
- Alphabetical is used
What is the general formula of a halogenoalkane
CnH2n+1 X
X represents the halogen
Describe the bond polarity of halogenoalkanes
- C-X bond is polar because halogens are more electronegative than carbons, covalent bond is closer to the halogen
-Polarity affects the physical properties and the reactivity of halogenoalkanes - This means the hydrogen atom has a slight negative charge whereas the oxygen has a slight positive charge
Describe bp of halogenoalkanes
Halogenoalkanes have a higher bp than their equivalent alkanes
and this is because alongside instantaneous dipole dipole forces, halegenoalkanes also have permanent dipole dipole forces due to their polar C-X bond
These bonds are stronger than instantaneous dipole moments, and these are the only bonds that alkanes have
Halegenoalkanes also have more electrons
Describe the solubility of the halogenoalkanes
- Polar CX bonds are not polar enough to make the halogenoalkane soluble in water
- Halogenoalkanes cannot form hydrogen bonds with water
- Halogenoalkanes are soluble in non polar solvents such as alchols
What is a nucleophile
- They have a lone pair of electrons or an electronegative atom
- They are negativley charged or electron rich ( S-)
Describe what happens during the nucleophilic substitution of chloromethane with a hydroxide ion as a nucleophile
- Chloromethane has an electron deficient carbon atom with a slight positive charge
- On the hydroxide ion, the oxygen atom has a lone pair of electrons and a negative charge
-The lone pair of electrons on the hydroxide ion are attracted to the electron deficient carbon atom on the choromethane - This lone pair is now donated to form a covalent bond to the electron deficient carbon atom
- This is formed to the opposite side of the halogen atom
- This is to reduce repulsion between the negative hydroxide ion and the negative halogen atom
Then draw a curly arrow from the lone pair of electrons to the electron deficient carbon atom
Now covalent bond between the carbon atom and the halogen breaks as the C atom can only form a maximum of 4 bonds, with a pair of electrons moving to the halogen atom - This is heterolytic fission
- This forms the alchol and a halide ion
What are the conditions for the hydrolysis of a halogenoalkane using hydroxide ion
Reagent- In potassium hydroxide solution
Conditions- In aqeous solution, heat under refux
Mechanism -Nucleophilic subsitution
-With a nucleophile -:OH
Define electronegativity
Ability of an atom to attract the electron density in a covalent bond
Which of the atoms in a C- X bond will be electron deficient or electron rich?
The carbon atom will be electron deficient making it delta positive/slightly positive S+
The halogen atom will be electron rich making it delta negative/ slightly negative S-
This is because the halogen is always more electronegative than the carbon atom
Describe the trend in bond polarities C-X in group 7
Decreases down the group
C-F most polar
C-I least polar
Describe why halogenoalkanes are insoluble in water
They have a large R group which is non polar, only able to form van deer waals not strong enough to be soluble in water
Describe trend in boiling point of halegenoalkanes down group 7
C-X bond will have a higher bp when the X is replaced with a halogen further down group 7
So increases down the group
This is because the size of the halogens increases, meaning they have more electrons so the van deer waals are stronger
How does the carbon hydrogen bond enpalthy influence the rate of reaction?
The reactivity of halogenoalkanes depends on the ease C-X bond breaks
A weaker bond will break easier
Meaning halogenoalkanes wih a lower enthalpy change are more reactive , react quicker
A stronger C-X bond is more stable meaning it reacts slower and has a higher bond enpalthy
This means reactivity of the C- X bond increases as you go down group 7
Describe the role of a nucleophile in a substitution reaction
They use their lone pair of negative electrons to form a new bond with a carbon atom
This is because carbon atoms are electron deficient ( S+)
They form a new covalent bond
What is the curly arrows role
To show the movement of electrons
It starts from a lone pair of electrons to the centre of a bond
Give the equation of nucleophilic substitution of chloromethane with a hydroxide nucleophile
CH3Cl + :OH- = CH3OH + Cl-
Give the equation of nucleophilic substitution of chloromethane with a cyanide nucleophile
CH3CL + -:CN = CH3CN + Cl-
Products are ethane nitrile and a chlorine ion
Give the equation of nucleophilic substitution with chloromethane and ammonia
CH3Cl + 2NH3 = CH3NH2+ NH4Cl
Products are methyl amine and ammonium chloride
Describe the conditions needed for nucleophilic substitution with a cyanide ion
Regent- KCN dissolved in ethanol/ water mixture
Conditions= Heating under reflux
Mechanism= Nucleophilic substitution
Type of reagent = nucleophile
What is a primary halegenoalkane
One carbon attached to the carbon atom adjoining the halogen
What is a secondary halegenoalkane
Two carbon atoms are attached to the carbon atom adjoining the halogen
What is a tertiary halegenoalkane
Three carbon atoms are attached to the carbon atom adjoining the halogen
Define hydrolysis
Defined as the splitting of the molceule by a reaction with water
Describe and explain a hydrolysis reaction of an iodoalkane
The iodoalkane will form a precipitate with the silver nitrate first because the C-I bond is the weakest meaning it reacts faster and is hydrolysed the fastest
How can rate of formation of precipitates be used to compare the reactivity of different alkanes
Aqeous silver nitrate is added to a halegenoalkane
This halide leaving group combines with a silver ion to form a silver halide precipitate
This precipitate only forms when the halide has left the halegenoalkane so rate of formation of the precipitate can be used to compare the reactivity of different halegenoalkanes
The quicker the precipitate formed the fatser the substitution reaction and the more reactive the halegenoalkane
Explain the conditions needed for nucleophilic substitution with an ammonia
Reagent- NH3 dissolved in ethanol
Conditions- Heating under pressure in a sealed tube
Mechanism- Nucleophilic subsitiution
Type of reagent- Ammonia nucleopile ;NH3
Describe the conditions needed for elimination with alcholic hydroxide ions
Reagent- Potassium or sodium hydroxide
Conditions- In ethanol and heat
Mechanism- Elimination
Type of reagent- Base OH-
Explain the differences in conditions for a elmination reaction and a substiution reaction
The solvent- Water encourages subsitutin and ethanol encourages elimination
Conc of the sodium hydroxide or potassium hydroxide- more conc encourages elimination, more dilute encourages subsitution
What is meant by heating under reflux
-Heating with a condenser placed vertically in the flask to avoid loss of volatiole liquids
Describe process of elimination
A hydroxide ion ( from sodium hydroxide or potassium hydroxide) acts like a base/ proton acceptor removes a hydrogen as a hydrogen ion from the halegenoalkane
This can only happen from a hydrogen from a carbon thats next to the carbon adjoining the C-X bond
This causes electron pair movements resulting in the formation of a double C=C bond
Also removes the halide ion as the approach of these electrons repels the electrons in the C-X bond
