Halogenoalkanes Flashcards
(32 cards)
What is the functional group?
C-X
What is the general formula?
CnH2n+1X
What are primary, secondary and tertiary halogenoalkanes?
Primary - One carbon attached to the carbon bonded directly to the halogen
Secondary - Two “”
Tertiary - Three “”
What do strong electronegativity values in C-F and C-Cl give rise to? except when what?
Permanent dipole-dipole forces of attraction. Except when the molecule is symmetrical then it is non-polar.
What forces of attraction are between all halogenoalkanes?
Van der Waals
Why does 1-chlorobutane have a higher m.p than hexane
Hexane only has Van der Waals whereas 1-chlorobutane has both permanent dipole-dipole and Van der Waals
From chloromethane to Iodomethane what happens to the b.p?
Increases because the number of electrons increases, therefore Van der Waals increases.
What happens to the b.p when the carbon chain increases
Increases because the number of electrons increases therefore the number of induced dipoles increases and Van Der Waals increases
What happens when it becomes branched?
Becomes slightly less as they are unable to make as much contact with eachother.
What are some other properties of halogenoalkanes
1) They have a sweet, sickly smell
2) They are immiscible with water and form two distinct layers.
How are halogenoalkanes prepared
Prepared by reacting an alcohol with a hydrogen halide.
Show the preparation of 1-bromobutane from butan-1-ol
CH3CH2CH2CH2OH + HBr –> CH3CH2CH2CH2Br + H2O
How is the Hydrogen bromide generated?
In situ from the reaction of sodium bromide with concentrated sulfuric acid.
NaBr + H2SO4 → HBr + NaHSO4
What are the three types of reactions that produce halogenoalkanes?
1) From alkanes
Alkane + Halogen → Halogenoalkane + Hydrogen halide
Conditions - UV light
Mechanism - Free radical photochemical substitution
2) From alkenes
Alkene + Hydrogen halide → Halogenoalkane
Conditions - none
Mechanism - electrophilic addition
3) From alcohols
Alcohol + HX → Halogenoalkane + H2O
Conditions - HBr generated in situ from NaBr and H2SO4
Mechanism - Substitution
What happens when a halogenoalkane is reacted with an alkali, OH-?
It forms an alcohol + the halogen
What are the conditions necessary for a halogenoalkane to react with an alkali
Heat under reflux with aqueous Sodium Hydroxide and a small quantity of ethanol for 1 hour
What is the mechanism?
Nucleophilic substitution / hydrolysis
How do you acid hydrolyse a halogenoalkane? Whats the equation?
Heat the substance under reflux with dilute mineral acid.
With H+ written above the arrow
C2H5Br + H2O → C2H5OH + HBr
How do you base catalyse a halogenoalkane? Whats the equation?
Heat the substance with dilute alkali
C2H5Br + NaOH → C2H5OH + NaBr
What is the mechanism for the above two reactions?
Nucleophilic substitution
What is the order of bond length and what does this mean?
C-I In order of decreasing bond length
C-Br
C-Cl
This means C-I bonds will hydrolyse more easily than C-Br and C-Br more easily than C-Cl.
How do you demonstrate this experimentally?
Add 1㎤ of each halogenoalkane to a test tube, add 2㎤ of ethanol and 1㎤ of silver nitrate solution to each. Place them into hot water. First a yellow ppt, then a cream ppt, the lastly a white ppt. Yellow = I, Cream = Br and White = Cl
What is the equation for the reaction of a halogenoalkane with ammonia
Forms an amine and a hydrogen halide
C2H5Br + NH3 → C2H5NH2 + HBr
What conditions are required ?
Heated with ethanolic ammonia in a sealed tube
