Halogenoalkanes Flashcards
Describe how boiling points change in a halogenoalkane
- It increases with increased chain length as there are more Van der waals forces
- Haloalkanes have higher boiling points than hydrocarbon chains of similar lengths due to greater relative molecular mass and polarity (dipole dipole)
Describe how bond enthalpy changes in a halogenoalkane
Bond enthalpy decreases as you go down the group
Attractive forces between shared bonding pair and nucleus of halogens is weaker as you go down the group
This causes reactivity to increase as you go down (easier to break)
Bond polarity
The C-X bond in a haloalkane is POLAR because the halogen(X) is more electronegative
Solubility of haloalkanes
•The polar C-X bonds are NOT POLAR ENOUGH TO MAKE THE HALOALKANE SOLUBLE IN WATER
•Main intermolecular forces are van der waals and dipole dipole (no hydrogen bonds)
Why does boiling point decrease when the branching of chains increases
There is less interaction between molecules because they can’t get as close together decreasing the area of contact which weakens VDW forces
What are the conditions of nucleophilic substitution with OH-
In a warm aqueous sodium hydroxide solution
heated under reflux
forms an alcohol
What are the conditions of nucleophilic substitution with CN^-
Warm Ethanolic potassium cyanide mix and heated under reflux
Conditions of nucleophilic substitution with NH3
Heat with ethanolic ammonia
Must have an excess of ammonia
Why does the carbon halogen bond mentally influence the rate of reaction
The carbon halogen bond has to be broken during the reaction. the harder the bond is to break (the higher the bond enthalpy) the slower the haloalkanes reacts
Why does the mechanism of nucleophilic substitution with NH3 require two moles
One acts as a nucleophilic and the other acts as a base
Conditions for the elimination reaction of a haloalkane with sodium or potassium hydroxide and mechanism for it (X=Halogen)
Elimination
Role of hydroxide ions is base
Warm Ethanolic sodium or potassium hydroxide and heat under reflux
Arrow from Lone pair on OH- to Hydrogen adjacent to the C-X
Arrow from the hydrogen bond line thingy to the carbon bond line thingy
Arrow from C-X to the X
Forms an alkene
Overall reaction: CH3CHXCH3 + KOH–>CH2=CHCH3+ H2O +KX
What happens when a reagent acts as a nucleophilic
it acts as an electron pair sonar and a substitution takes place
What happens When a reagent acts as a base
it acts as a hydrogen ion acceoter and an elimination takes place
Do primary HA mainly undergo elimination or subsititution
Subsistuition
Do secondary haloalkanes mainly undergo elimination or Subs
Both
Do tertiary HA mainly undergo elimination or subs
Elimination
What does water encourage
Substitution
formation of alcohol
What does ethanol encourage
elimination
formation of alkene
A higher temperature encourages
Elimination
A higher conc of potassium or sodium hydroxide solution encourages
Elimination
What is a nucleophile
An electron pair donor
e.g. Ammonia, hydorxide ions and cyanide ions (have to show charges on the ions and at leats onelone pair on each)
Steps of the mechanism for hydroxide ions
arrow from the lone pair to the carbon attached to the halogen
arrow from the c-halogen bond to the halogen
forms an alcohol + halogen with a lone pair of electrons drawn and an anion
Overall reactionR-X +NaOH–>ROH+NaX
(X is halogen R is the carbon chain
Steps of the mechanism for cyanide ions
arrow from the lone pair to the carbon attached to the halogen
arrow from the c-halogen bond to the halogen
forms an nitrile + potassium halogen with a lone pair of electrons drawn and an anion
Overall reactionR-X +KCN–>RCN+KX
(X is halogen R is the carbon chain
Steps of the mechanism for ammonia (n+= aminocation)
arrow from the lone pair to the carbon attached to the halogen
arrow from the c-halogen bond to the halogen
forms an aminocation
Another molecule of ammonia arrow from lone pair of that ammonia to the hydrogen attached to the aminocation arrow from H-N to the N+ produces an amine(NH2)
Overall reaction: CH3CH2Cl +2NH3 –> CH3CH2NH2 + NH4Cl
