organic chemistry haloalkanes Flashcards
what is a halogenalkane
a compound which has an R–X R (alkyl group eg. alkane or alkene chain) X (any halogen atom)
what is the general formula for a halogenalkane
monosubstituted : CnH2n+1+X
explain why there is bond polarity of halogenalkanes
have a C—X bond
This is a polar bond with the Carbon being delta positive (electron deficient) and the halogen being delta negative
As go down group of halogens they get less polar as difference in electronegativety decreases
explain the different types of halogenalkanes
can be classified as primary, secondary or tertiary
depends upon the number of alkyl groups attached to the carbon which is bonded with the halogen
explain the trend in boiling points with halogenalkanes
the boiling point of simple molecular substances depend on the strength of the intermoleular forces
Halogenalkanes form dipole-dipole and van der waals
strength depends on
: increase BP with increase in carbon chain length
: increase BP as size of halogen increases
: decrease in BP with increase in branching
have higher boiling points than alkanes as they have higher relative masses and are more polar with permanent dipole dipole forces
explain the solubility of halogenalkanes
the C—-X bonds are not polar enough to make them souluble in water as the majority of the molecule is not polar
tend to be more soluble in non-polar solvents eg. hexane
what are the two factors that determine how halogenalkanes react
almost always the C—X bond that breaks
two main factors that determine how easily this bond breaks
1) bond polarity
2) bond enthalpy
explain the polar bonds with haloalkanes with susceptibility to an attack from a nucleophile
The high electronegativity of the halogen atom causes the carbon to become electron deficient and susceptible to an attack from a nucleophile
Nucleophile is a species that is a electron pair donor
This theory would predict C-F bond would be the most reactive as most polar and susceptible to an attack from a nucleophile
explain how bond enthalpy explains the reactivity of the haloalkanes
The bond enthalpy decreases as you go down the group
this is because the halogen atom is larger meaning the C–X bond must be longer making it weaker
This predicts that iodo compounds must be the most reactive as they have the longest and weakest bonds and floro compounds are the least reactive
experiments confirm that reactivity increases going down the group, this means that bond enthalpy is a more important factor than bond polarity
what is nucleophilic substitution
Haloalkanes are susceptible to attack of nucleophiles (electron pair donors) such as OH- CN- and NH3
in a substitution reaction the halogen atom is replaced by another atom/group
show an example of NS with warm aqueous NaOH
reagents : warm aqueous NaOH (+haloalkane)
conditions : warm aqueous
Nucleophile : hydroxide ion :OH-
what happens : halogen atom is replaced by OH
overall equation : R—–X + NaOH → R—–OH + NaX
mechanism :
show the equation and mechanism for 2-iodo-3-methylbutane with aqueous NaOH
show an example of NS with KCN
reagents : warm aqueous ethanoic potassium cyanide (KCN) (+haloalkane)
conditions : warm aqueous ethanoic
Nucleophile : cyanide ion :CN-
what happens : halogen atom is replaced by CN
overall equation : R—–X + KCN → R—–CN + KX
mechanism :
show the equation and mechanism for 1-bromobutane and ethanolic KCN
show an example of NS with NH3
reagents : ammonia (NH3) (+haloalkane)
conditions : excess concentrated NH3 dissolved in ethanol, under pressure in a sealed container
Nucleophile : ammonia :NH3 , 2 molecules
what happens : First M: halogen atom is replaced by a NH2
second M: leads to the formation to NH4X
show the equation and mechanism of 2-bromo-3-methylbutane in an excess of conc NH3
explain elimination mechanisms with haloalkanes
if reaction conditions are changed they can react to form alkenes
In this elimination reaction the hydroxide ion acts as a base to remove a proton (H+) from the alkane , the electron in the C-H bond double between the C that has lost the proton and the C atom which is attached to the halogen
show and explain the mechanism and equation of NaOH and KOH in ethanol
reagents : NaOH/KOH
conditions : ethanolic conditions + hot reflux
what happens : halogen atom and one hydrogen atom from the adjacent C atom are removed giving an alkene
- elimination can not happen if there is no hydrogen on a adjacent C
show and explain the mechanism of 2-chloropropane and hot ethanolic KOH
show the mechanism of 2-bromobutane + hot ethanolic KOH to give but-2-ene
explain how the conditions can determine what kind of haloalkane is made
explain how elimination mechanisms can give different products depending if the halo alkane is primary, seconday or tertiary
what is the ozone layer
The ozone layer is found in the stratosphere, ozone molecules O3 are present in high concentrations and continually formed and broken down by intense short wavelength UV radiation. They reduce amount of harmful radiation that reaches earth responsible for sun burn, skin cancer etc
show and explain the two step reaction to form ozone
1) oxygen molecules O2 are exposed to UV light causing homolytic bond breaking to happen forming 2 oxygen free radicals (species with unpaired e)
O2 → 2O•
2) an oxygen free radical reacts with an oxygen molecule (O2) to form ozone (O3)
O• + O2 → O3
explain the break down of ozone
the breakdown occurs on the absorption of UV light (at the wavelength that damages biological life). This forms an oxygen molecule (O2) and a oxygen free radical
O3 → O2 + O•
without ozone depleting pollutants in the atmosphere the concentration of ozone would remain at constant as it is formed at the equal rate as its broken down
explain and describe what CFCs are
CFCs are chloroflorocarbons, they all have the hydrogen atoms replaced by either a chlorine or a fluorine. They are used for anasthetics, refrigerants and aerosol propellants as they are unreactive under standard conditions and are volatile
examples include trichlorofluoromethane and 1,1,2-trichloro-1,2,2- trifluoroethane
explain and describe how CFCs have aa detrimental effect on the ozone layer
CFCs are unreactive as the C-F and C-Cl bonds are relatively strong, therefore they can exist in the atmosphere
when these molecules reach the stratosphere the high energy UV light from the sun provides enough energy to break the C-Cl bond homolytically, prefer over C-F as C-Cl bond is weaker
show and explain the overall equation for the decomposition of ozone
The chlorine free radical is regenerated (acts as a catalyst) and goes on to react further. Allows the breakdown of ozone via an alternate reaction pathway of lower activation energy.
explain what we use for replacements of CFCs
hydrofluorocarbons (HFC) are noew used
Only contain fluorine not Cl
C-F bond is much stronger than the C-Cl so are less reactive, not broken down in the stratosphere as the UV light does not provide sufficient energy and a free radical is not produced
