Topic 10 Flashcards
What are the main features of a homologous series?
- gradual change in physical properties
- successive members differ by a CH2 unit
- SIMILAR chemical properties because they have the same FUNCTIONAL GROUP
- same general formula
Describe primary, secondary and tertiary carbon atoms
primary - bonded to 0 or 1 other carbon atom
secondary - bonded to 2 other carbon atoms
tertiary - bonded to 3 other carbon atoms
CARBON CAN ONLY FORM 4 bonds
What is benzene’s functional group?
Phenyl group
has the formula C6H5
What does the 1:1 hydrogen carbon ratio in benzene indicate?
A high degree of saturated, greater than alkenes and alkynes
Why doesn’t benzene undergo addition reactions?
electrons of benzene ring are delocalised through out the molecule, which makes the molecule very stable. The addition reaction would result in breaking of this delocalisation so the stability of molecule is destroyed
Describe homolytic bond fission
A covalent bond between the 2 atoms in a molecule breaks, with each atom taking one electron from the bond
Results in formation of free radicals which are highly reactive species with unpaired electrons
Describe heterolytic bond fission
A covalent bond between two atoms in a molecule breaks with one atom taking both bonding electrons
Describe kekulé structure of benzene
Alternating carbon to carbon single and double bonds
Actual structure of benzene is a resonance hybrid with equal bonds that are intermediate in length and strength between a single and double bond
Why don’t alkanes undergo many reactions?
unreactive
strong covalent bonds that require a lot of energy to break
C-H bond is non-polar
Describe free radical substitution of alkanes
Alkanes undergo free radical substitution reactions
This is when an atom or group of atoms are replaced by another atom or group
most common type is halogenation
Describe steps of substitution of bromine in methane
Initiation:
- occurs in the prescence of UV light
- photochemical homolytic fission occurs when halogen is broken by UV into 2 halogen radicals
Propagation:
- First, free bromine radical reacts with methane to produce a methyl radical (Ch3.) and hydrogen bromide (as the hydrogen binds to bromine radical)
- then, methyl radical produced in first propagation step will react with a bromine molecule to produce bromomethane and a bromine radical
- methyl radical reacts with bromine to produce a bromine radical and bromomethane
Termination:
- free radicals react with each other to form molecules. Reaction stops when there are no more radicals
Test for unsaturation
bromine water tests for alkenes as alkenes have a double bond
brown to colourless
addition reaction occurs and alkane is formed with 2 bromine functional groups
Describe hydrogenation
An alkene reacts with hydrogen to form an alkane
The C=C bond is broken
2 individual H atoms attach to carbon
Nickel catalyst used
Hydration
When an alkene reacts with steam to produce an alcohol
Double bond breaks and H20 attaches itself as H and OH onto the carbon atoms that are now open
Catalyst: sulfuric acid
Halogenation
Dihalogeno compounds formed
Halogen replaces 2 hydrogens
halogenoalkane formed
Describe the reactions of the alcohols
Oxidation:
- used to make aldehydes, ketones and carboxylic acids
- primary alcohols oxidized to form aldehydes and oxidized again to form carboxylic acids
- secondary alcohols are oxidized to form ketones
- tertiary alcohols not oxidized because alcohol group has 0 hydrogens attached to it
Combustion:
alcohol + oxygen - water and co2
Esterification:
- react with a carboxylic acid to form an ester
Describe esterification
Esters are formed when carboxylic acids react with alcohols in the prescence of sulfuric acid as a catalyst
Describe the evidence that benzene does not have 3 double bonds
Bond enthalpy is -120, would have been -360 if 3 doubles bonds
All C-C bonds same length, implying intermediate length due to resonance
Only 1 isomer exists: 1,2-disubstituted benzene
Benzene does not readily undergo addition reactions because it is stable with its resonance structure
Describe change in boiling point in homologous series
Boiling point increases as carbon chain increases, LDF increases
rate of increase decreases as molecules become more branched and more spherical, reducing surface area
Define structural isomerism
Structures with the same molecular formula but different structural formula.
Similar chemical properties but physical properties may be different
Why does combustion of alcohols release less energy than combustion of alkanes
It is partially oxidised
Describe oxidation of ethanol
Ethanol is readily oxidized by warming with an acidified solution of potassium dichromate (VI)
Dichromate ion is reduced from oxidation state of +6 to Cr3+
ethanol oxidised to ethanal first
unlike ethanol and oic acid, ethanal does not have hydrogen bonding so has a lower boiling point. To stop the reaction at the aldehyde stage ethanal can be distilled from the reaction
ethanal oxidized further to ethanoic acid, mixture heated under REFLUX to ensure no ethanal escapes
Describe oxidation of primary, secondary and tertiary alcohols
ALL primary alcohols are oxidized by acidified potassium dichromate (VI) first to aldehydes, then to carboxylic acids
secondary alcohols oxidized to ketones which cannot undergo further oxidation
tertiary cannot be oxidised as they have no hydrogen atoms directly attached to carbon containing functional group. Burn readily though
Alkene to halogenoalkane process + catalyst?
- hydrogen halide
- addition reaction
Alkene to alkane process and catalyst?
- Heat
- Hydrogen
- Nickel catalyst
- halogenation
Alkane to halogenoalkane?
- Free radical substitution
- halogen
UV light
Alkene to alcohol?
- hydration
- Concentrated Sulphuric acid
- water
Halogenoalkane to alcohol?
- nucleophilic substitution
- nucleophile
Alcohol to carboxylic acid?
Oxidation of primary alcohols, Kr2Cr2O7, H+ and distillation and then reflux from aldehyde
Carboxylic acid to ester?
- alcohol
- concentrated sulphuric acid
- heat
Alcohol to ester?
- carboxylic acid
- heat
- concentrated H2So4
Ketone to alcohol
NaBH4/methanol
Aldehyde to alcohol
NaBH4/methanol
Explain the production of nitrobenzene
- Concentrated H2So4 and HNO3 react to product H2O, HSO4- and No2+ (our nucleophile)
- NO2+ is attracted to delocalised pi electron system of stable benzene
- forms bond with benzene, a pair of electrons given to oxygen of the No2+ nucleophile
- Stable delocalised pi electron system disrupted
- HSo4- pulls hydrogen away, forming H2So4
- C-H bond breaks after H donates its electrons to restore the stability of benzene, replacing electrons lost that were stolen by nucleophile bonding to benzene
- End products are H2So4 and nitrobenzene
Production of phenylamine from nitrobenzene
Concentrated Sn and HCl react with nitrobenzene to form phenylammonium under reflux conditions in a boiling water bath
phenylammonium reacts with NaOH to form the phenylamine molecule, by removing H+
Cis/trans rotation?
NO free rotation
When do you use E/Z isomerism?
When there are more than 2 different substituent groups
What is a chiral compound
A compound that has a carbon atom bonded to 4 different groups
What is an enantiomer
chiral compounds that cannot be superimposed onto each other
What is a racemic mixture
A substance containing equal amounts of both enantiomers
How to test for a racemic mixture
Shine a plane polarised light onto substance
The enantiomer will rotate the PP light by a certain amount and by certain degrees
Other enantiomer will also rotate PP light by same amount but in the opposite direction
so net rotation is 0.
Racemic mixture does not rotate plane polarized light
When does cis/trans isomerism occur?
Both cis and trans and E/Z occur when rotation about a bond is restricted or prevented.
When there is a single bond between two carbon atoms, free rotation about the bond is possible however
double bond is made up of 1 sigma and 1 pi bond.
The pi bond is formed from the combination of 2 p orbitals, one from each of the carbon atoms. These 2 p orbitals must be in the same plane to continue.
Rotating the bond would cause the pi bond to break so no rotation possible.
Explain the physical and chemical properties of E/Z and cis-trans isomers
Cis has higher boiling point than trans.
In the cis-isomer, the two carboxylic groups are closer together, allowing strong hydrogen bonds but in trans, too far apart so no hydrogen bonding
The cis-isomer reacts when heated to lose water whereas trans cannot undergo this
Describe optical isomerism
OI is shown by at least 1 asymmetric or chiral carbon atom within the molecule.
The two isomers are known as enantiomers and are mirror images, cannot be superimposed
The two isomers are optically active with plane polarized light
Describe the difference between plane polarized light and normal light
Normal light consists of electromagnetic radiation which vibrates in all planes. When it is passed through a polarizing filter, the waves only vibrate in one plane and the light is said to be plane-polarized
Explain the effect of enantiomers on plane polarized light
One of the enantiomers rotates it to the left and other rotates it to right by the same amount. Apart from their behaviour towards PP light, they have IDENTICAL physical properties.
Chemical properties are identical too except when they interact with other optically active substances
When does diastereomerism occur?
When 2 or more stereoisomers of a compound have different configurations at one or more of the equivalent stereocentres.
Explain the role of a polarimeter
Used to measure optical activity of enantiomers. It consists of a light source, two polarizing lens and between the lenses a tube to hold the sample of the enantiometer dissolved in a suitable solvent
When light passes through the first polarizing lens, it becomes plane polarized, meaning it vibrates in a single plane
With no sample present, the observer will see the max intensity of light when the second polarizing lens is in the same plane
When sample is placed between the lenses, the analyser must be rotated by 0 degrees either clockwise or anticlockwise to give light of max intensity
The two enantiometers rotate the plane of plane-polarized light by the same amount but in opposite directions. If both enantiometers are present in equal amounts, two rotations cancel each other out and it is a RACEMIC mixture
Explain the solubility of compounds in water
The solubility of organic compounds in water depends on their ability to form hydrogen bonds with water molecules.
Compounds that are able to form hydrogen bonds with water molecules will be soluble in water, although the non-polar hydrocarbon part of the molecule does interfere with this ability.
The first three alcohols - methanol, ethanol and propanol - are considered to be completely soluble in water. As the length of the hydrocarbon chain increases, however, the solubility decreases.
Alcohols with 10 or more carbon atoms, such as decanol (CH3(CH2)9OH), are considered to be insoluble in water.
This is due to the increasing size of the hydrocarbon chain, which increases the hydrophobic character of the molecule, decreasing its solubility in polar solvents such as water
Define volatility
The measure of how easily a substance evaporates is referred to as its volatility.
A highly volatile substance has a low boiling point and evaporates easily.
Substances that have stronger intermolecular forces are less volatile and consequently have higher boiling points.
What affects the boiling point and volatility of organic compounds
Firstly, the boiling point of an organic compound increases with increasing molar mass. As a consequence of the increasing molar mass, there are stronger London dispersion forces between the molecules, which result in an increase in the boiling point
Secondly, branched-chain isomers usually have lower boiling points than the corresponding straight-chain isomers. The branching of a chain produces a more spherical shape to the molecule. This results in less surface contact between the molecules than with straight-chain isomers. Therefore, branched-chain isomers have weaker intermolecular forces and, consequently, lower boiling points
Finally, the nature of the functional group influences the boiling point and volatility of an organic compound. Polar functional groups result in stronger dipole–dipole interactions between the molecules, and therefore higher boiling points. Compounds with functional groups that contain O-H or N-H bonds are capable of forming hydrogen bonds between their molecules.
There are three positional isomers of pentanol: pentan-1-ol, pentan-2-ol and pentan-3-ol. In this order, you would expect the strength of the intermolecular forces to….?
The presence of the hydroxy group introduces strong hydrogen bonds between molecules.
The position of the hydroxy group affects the strength of the hydrogen bonding.
The primary alcohol (pentan-1-ol) contains the hydroxy group on the end of the molecule, so it has a strong net dipole.
For hydroxy groups farther away from the terminal carbon, the net dipole decreases, lowering the strength of the intermolecular forces and the boiling point.
Rank the following molecules by increasing order of pH, separated by commas.
methylamine, methanoic acid, methane
The functional group determines the acid–base behaviour of the molecule. Methanoic acid contains one ionisable hydrogen, making it acidic, and thus it has the lowest pH. Methane does not have any acid–base properties, so is neutral. Methylamine has a lone pair on the nitrogen atom, so is capable of gaining a H+ ion, making it a base, and thus it has the highest pH
