4.2 Alcohols, haloalkanes and analysis Flashcards
alkanes vs alcohols polarity and MP/BP
alkanes = non-polar
non-polar bonds present as electronegativity of H and C very similar
alcohols = polar
contain polar O-H bond (high difference in electronegativity)
alcohols = higher MP/BP, lower volatility
hydrogen bond between O-H groups as well as London dispersion forces
requires more heat energy to overcome
alkanes vs alcohols solubility
alkanes = insoluble
no interactions with water (polar solvent)
alcohols = soluble
can interact with water (polar solvent)
due to polar -OH group
decreases as hydrocarbon chain gets longer (influence of sole -Oh group gets smaller)
classifying alcohols
primary alcohol = -OH group attached to carbon attached to just one alkyl group
secondary alcohol = -OH group attached to carbon attached to 2 alkyl groups
tertiary alcohol = -OH group attached to carbon attached to 3 alkyl groups
combustion of alcohols
alcohol + oxygen -> carbon dioxide + water
oxidation of alcohols
primary and secondary alcohols only
need an oxidising agent [O]
orange solution turns green
oxidation of primary alcohols
primary alcohol -> aldehyde -> carboxylic acid
conditions to prepare aldehyde
primary alcohol
oxidising agent
gently heat
distill at the same time to prevent further reaction with oxidising agent
conditions to prepare carboxylic acid
heated strongly
under reflux (ensures all of alcohol is oxidised)
excess of oxidising agent (e.g. potassium dichromate)
oxidation of secondary alcohols
secondary alcohol -> ketone
heated under reflux
oxidation of tertiary alcohols
nothing happens
doesn’t undergo oxidation reactions
dehydration of alcohols
heated under reflux
in presence of acid catalyst (e.g. concentrated H2SO4/H3PO4
alcohol -[catalyst]-> alkene + water
dehydration definition
water molecule is removed from starting material
type of elimination reaction
substitution reaction of alcohols
alcohol heated under reflux with sulfuric acid and sodium halide
NaBr(s) + H2SO4(aq) -> NaHSO4(aq) + HBr(aq)
hydrogen halide reacts with alcohol to produce haloalkane
alcohol + hydrogen halide -> haloalkane + H2O
nucleophile definition
(group of) atom(s) attracted to an electron deficient centre to form a new covalent bond, electron pair donor
why haloalkanes can attract nucleophiles
carbon-halogen bond is polar
carbon atom is partially positive (halogen pulls covalent pair away from carbon)
the centre of the haloalkane is electron deficient
hydrolysis of haloalkane
add aqueous sodium hydroxide
nucleophile (OH-) approaches back of carbon atom (attached to halogen) opposite side to halogen
lone pair of electrons on hydroxide attracted and donated to partially positive carbon atom
new bond formed between oxygen and carbon atom
carbon-halogen bond breaks by heterolytic fission
forms alcohol and halide ion
rate of hydrolysis and carbon-halogen bond strength
bond is weaker due to larger atomic radius of larger halide
lower bond enthalpy
the weaker the bond, the faster the hydrolysis (less energy required to break carbon-halogen bond)
measuring rate of hydrolysis of primary haloalkanes
complete the reaction in presence of AgNO3(aq)
Ag+ ions react with halide ions produced to form precipitate
also in presence of ethanol solvent (allows water and haloalkane to mix to form single solution)
water bath
why rate of reaction increases with haloalkanes with halogens lower down group 7
C-halogen bond becomes weaker with halogen lower down the group (larger atom, larger atomic radius, weaker attraction)
use of organohalogens
general solvent dry cleaning solvents making polymers flame retardants refrigerants
breakdown of ozone layer equations
CF2Cl2 → CF2Cl• + •Cl
•Cl + O3 → •ClO + O2
•ClO + O → •Cl + O2
overall equation: O3 + O -> 2O2
formation of ozone
O2 -> 2O •
O2 + O • ⇌ O3
what infra red does to covalent bonds
infra red causes covalent bonds to vibrate more and absorb energy
factors of what frequency of infrared is absorbed by covalent bonds
length of covalent bond
mass of atoms on either side of bond
why CO2, CH4 and H2O contribute to greenhouse effect
greenhouse gases
as C-H, O-H, C=O bonds absorb infrared radiation
uses of IR spectroscopy
identify types of bonds present in unknown substances
monitor gases causing air pollution (e.g. CO and NO from car emissions)
breathalysers (measure ethanol in breath)
fingerprint region
< 1500 cm^-1
contains unique peaks to find a particular molecule
molecular ion definition
clear peak furthest to the right on mass spectrum
M^+
positive ion formed when a molecule loses an electron
its mass-to-charge ratio (m/z) = Mr of original molecule
M + 1 peak
very small peak furthest to the right
due to larger isotope of carbon
fragmentation definition
molecular ions break down into smaller pieces
forms fragment ions and free radicals
only fragment ions shown on mass spectrum as it has positive charge (radicals uncharged so not detected)
oxidising agent foralcohols
acidified potassium/sodium dichromate
orange
how nitrogen (II) oxide formed
combustion in engines
lightning
N2(g) + O2(g) -> 2NO(g)
effect of nitrogen (II) oxide
acid rain
depletion of ozone layer
nitrogen (II) oxide reactions for depleting ozone layer
•NO + O3 -> •NO2 + O2
•NO2 + O -> •NO + O2
net reaction: O3 + O -> 2O2
how to purify organic liquid
separate aqueous layer from organic layer using separating funnel
dry using anhydrous salt (e.g. MgSO4, CaCl2)
redistillation
esterification
when carboxylic acid and alcohol react to form ester + water
esterification conditions
carboxylic acid group + hydroxyl group
concentrated sulfuric acid catalyst
anhydrous salts examples
anhydrous:
CaCl2
MgSO4
CaSO4
