halogenoalkanes Flashcards
What do halogenoalkanes contain
Polar bonds
How do you name halogenoalkanes
The halo part of the name comes first, along with a number to indicate where the halogen is -(if needed). If there is more than one type of halogen put the prefixes in alphabetical order.
How can haloalkanes be classified
o Primary
o Secondary
o Tertiary
Why is something a haloalkane
The Haloalkanes are alkanes that contain a halogen, F, Cl, Br, I
One or more H atoms have been replaced by a halogen
What is a primary haloalkane
• In a primary haloalkane the halogen is attached to a C, which is only attached to one other C.
What is a secondary haloalkane
• In a secondary haloalkane the halogen is attached to a C, which is only attached to two other C’s.
What is a tertiary haloalkane
• In a tertiary haloalkane the halogen is attached to a C, which is attached to three other C’s.
An alkane with a halogen needs what to form into a haloalkane
Uv light
What is the reaction called that goes from an alkane to a halo alkane
Substitution reaction
Define a free radical
A free radical is a species which has a single unpaired electron.
Cl2 -> 2Cl .
Why is UV light needed to start the reaction?
UV light provides the energy needed to break THE Cl-Cl bond.
What is homolytic fission
Cl2 -> 2Cl .
Both Cl atoms have donated one pair of electrons into the covalent bond, and when the bond breaks they each take back their electron.
Define a Nucleophile
Can donate a lone pair of electrons, a lone pair donor
E.g OH–, CN– and NH3
What is a Nucleophilic substitution
The replacement of a halogen by a nucleophile
What are the 3 reactions of the haloalkanes
Nucleophilic substitution by hydroxide ions (OH-, hydrolysis)
Nucleophilic substitution by cyanide ions (CN-)
Nucleophilic substitution with ammonia (NH3)
Haloalkane -> Nitrile
What is the reaction
What is the reagent
What is the condition
Nucleophilic substitution by cyanide ions (CN-)
KCN
Water & ethanol
Haloalkane -> Amine
What is the reaction
What is the reagent
What is the condition
Nucleophilic substitution with ammonia (NH3)
NH3
Excess
Haloalkane -> Alcohol
What is the reaction
What is the reagent
What is the condition
Nucleophilic substitution by hydroxide ions (OH-, hydrolysis)
NaOH
Aqueous solvent, heat under reflux
What is the nucleophilic substitution of a haloalkane using hydroxide ions called
Hydrolysis
Explain why the carbon–halogen bond enthalpy influences the rate of reaction
Bond Strength (kJ mol-1)
C-F -> + 467
C-Cl -> +340
C-Br -> +280
C-I -> +240
As the bond enthalpy (strength or heat energy needed to break a bond) decreases it is easier to break the
C – Halogen bond.
The stronger the bond the harder it is to break and the slower the rate of reaction will be as more energy is required to break it.
The weaker the bond the faster the rate will be.
Explain why the c-I bond is the weakest and the C-Cl bond is the strongest
Bond Strength (kJ mol-1)
C-F -> + 467
C-Cl -> +340
C-Br -> +280
C-I -> +240
C – I- weakest bond so fastest reaction, less energy require to break the bond
C – Cl - strongest bond so slowest reaction, more energy required to break the bond
Where is ozone formed naturally
Ozone is formed naturally in the upper atmosphere (stratosphere) by free radical reactions
What are replacements for CFC’s which are chlorine free
Alkanes, HFC’s hydrofluorocarbons and HCFC’s hydrochlorofluorocarbons are used frequently.
Carbon dioxide is used as a replacement for blowing agents for expanded polymers (as in polystyrene).
Where are chlorine atoms formed and how
formed in the upper atmosphere when ultraviolet radiation causes C–Cl bonds in chlorofluorocarbons (CFCs) to break
How do CFC’s damage the ozone layer
Spec - Cl. Acts as a catalyst in the decomposition of ozone. One Cl. Radical breaks down many ozone molecules.
What are the useful properties of CFC
• non-toxic
• inert gases
• volatile
What are the replacements for CFC’s
Trichlorofluoromethane -> Refrigeration Air conditioning
Dichlorodifluromethane -> aerosol propellants, blowing agent for polystyrene
1,1,1 trichloro 2,2,2 trifluoroethane -> dry cleaning agents, degreasing agents for circuit boards
How does chlorine atoms catalyse decomposition of ozone
What are the two equations you need to know
Spec based - Cl• + O3 → ClO• + O2 and ClO• + O3 → 2O2 + Cl•
What is the role of the hydroxide ion in an elimination reaction
A base, as it is removing a hydrogen from the molecule
Haloalkane -> Alkene
What is the reaction
What is the reagent
What is the condition
Elimination
KOH or NaOH
Ethanolic, reflux
