C13 - Halogenoalkanes Flashcards
What are halogenoalkanes?
Halogenoalkanes are similar to alkanes however 1 or more of the hydrogen atoms have been replaced by a halogen atom. Halogenoalkanes can contain more than one type of halogen
What is the name of a halogen with one R group attached to the carbon linked to the halogen?
Primary (1°) halogenoalkane
What is the name of a halogen with two R group attached to the carbon linked to the halogen?
Secondary (2°) halogenoalkane
Are halogenoalkanes soluble in water?
No
What are the intermolecular forces found in halogenoalkanes?
Van der Waals and permanent dipole-dipole intermolecular forces
Why does the boiling point for a halogenoalkane with iodine compared to chlorine increase?
Stronger permanent dipole-dipole forces because of the more polar the bond being is a slight impact. However the main reason for an increase in boiling point is the increasing electron number meaning stronger Van der Waals forces
What happens to the boiling point the longer the chain length and why?
The intermolecular forces of attraction due to van der Waals’ forces increases as the molecular mass increases. Therefore, the boiling point of each halogenoalkane increases as the chain length increases
What occurs in free radical substitution of an alkane in synthesis of halogenoalkanes, using chlorine and methane as an example?
- In theinitiation step, the halogen bond (E.g. Cl-Cl) is broken by UV energy to form two free radical species (2Cl-)
- The radicals create further radicals in a chain reaction called thepropagation step
The chlorine free radical takes a hydrogen atom from methane to form hydrogen chloride, a stable compound. This leaves a methyl free radical, ·CH3
·Cl + CH4 🡪 HCl + ·CH3 ( · represents an unpaired electron)
The very reactive ·CH3 reacts with a chlorine molecule producing another chlorine free radical and a molecule of chloromethane, a stable compound
·CH3 + Cl2 🡪 CH3Cl + ·Cl
These steps may take place thousands of times (chain reaction) before the free radicals are destroyed in the termination step - The reaction ends when two radicals collide with each other in atermination step
·Cl + ·Cl 🡪 Cl2
·CH3 + ·CH3 🡪 C2H6
·Cl + ·CH3 🡪 CH3Cl
What are free radical species?
Free radical species – unstable chemical species with an unpaired electron
Why have chlorofluorocarbons(CFCs) been replaced as solvents and refrigerants?
As the CFCs are broken down by UV light causing the formation of chlorine radicals. These radicals react with ozone and break down the ozone layer
What are the reactions of chlorine free radicals with ozone and the overall equation?
·Cl + O3 🡪 ·ClO + O2
·ClO + O3 🡪 2O2 + ·Cl
To give an overall equation of 2O3 🡪 3O2
How does nucleophilic substitution of alcohols work with halogenoalkanes?
The nucleophile in this reaction is the hydroxide, OH-ion. Reaction mixture is warmed to increase rate of reaction
OH- attack the carbon of a carbon–halogen (C–X) bond, because the lone electron pair on the nucleophile is attracted to the small positive charge on the carbon. The electrons in the C–X bond are repelled as the nucleophile approaches the carbon atom. The OH- bonds to the carbon and the two electrons from the C–X bond move to the halogen, forming a halide ion. The halogen is substituted by the OH-, creating an alcohol
How does nucleophilic substitution of cyanide work with halogenoalkanes?
The nucleophile in this reaction is the cyanide, CN-ion.
CN- attack the carbon of a carbon–halogen (C–X) bond, because the lone electron pair on the nucleophile is attracted to the small positive charge on the carbon. The electrons in the C–X bond are repelled as the nucleophile approaches the carbon atom. The CN- bonds to the carbon and the two electrons from the C–X bond move to the halogen, forming a halide ion. The halogen is substituted by the CN-, creating a nitrile
How does nucleophilic substitution of ammonia work with halogenoalkanes?
Thenucleophilein this reaction is theammonia, NH3molecule
The lone pair of electrons of an ammonia molecule attacks the carbon atom
The lone pair of electrons of another ammonia molecule attacks one of the hydrogens attached to the nitrogen
What happens in an elimination reaction of a halogenoalkane?
The lone pair on the base attacks one of the hydrogens adjacent to the carbon with the halogen attached. The electrons in the C-H bond are pushed to make a C=C bond, the electrons on the C-halogen are repelled onto the halogen. Resulting in an alkene, water and a halogen ion to be formed
