Module 6: Organic Chemistry and Analysis Flashcards
What is benzene?
The simplest aromatic hydrocarbon (arene).
C6H6
It is a colourless, sweet-smelling, highly flammable liquid, which is classed as a carcinogen and found naturally in crude oil.
What is an aromatic compound?
Benzene derivatives (contain a benzene ring)
What are the two models for benzene’s structure?
- Kekule model: alternating single and double C-C bonds
- Delocalised model: each C atom shares the 6 delocalised electrons in a ring above and below the plane (sideways overlap of p orbitals)
What is the more widely accepted model of benzene and why?
Delocalised model. Kekule is not accepted because:
- Lack of reactivity: benzene doesn’t undergo electrophilic addition reactions and doesn’t decolourise bromine under normal conditions, which it should if it had C=C bonds
- Lengths of C-C bonds: X ray diffraction was used to find that all the C-C bond lengths in benzene are equal, which would not be the case if it contained both C-C and C=C
- Hydrogenation enthalpies: if benzene had the Kekule structure, it would be expected to have an enthalpy of hydrogenation 3x cyclohexene, but the actual enthalpy is less than expected, so the actual structure is more stable than the Kekule model
Describe the delocalised model of benzene
- Benzene is a planar, cyclic hydrocarbon.
- Each carbon uses 3 of its 4 electrons to bond to 2C and 1H atoms.
- Each C atom has 1 electron in a p orbital at 90 degrees to the plane of atoms.
- Adjacent p orbitals overlap sideways in both directions above and below the plane forming rings of electron density.
- Overlapping of p orbitals creates a system of pi bonds which spread over all 6C atoms in the ring.
- The 6 electrons in the pi bond system are said to be delocalised.
Describe briefly how you would name aromatic compounds
Most substituent groups are prefixes to benzene, including NO2 (nitro). When a benzene ring is attached to an alkyl chain with over 7 C atoms, benzene is the substituent and phenyl- is used as a prefix.
What are the 3 aromatic compounds whose names need to be learnt?
- Benzoic acid (COOH attached to a benzene ring)
- Phenylamine (NH2 attached to a benzene ring)
- Benzaldehyde (CHO attached to a benzene ring)
What type of reaction does benzene typically undergo?
Electrophilic substitution
Describe the nitration reaction of benzene
Benzene + HNO3 —> nitrobenzene + H2O
Uses H2SO4 catalyst at 50 degrees, at higher temperatures further substitution may occur
Give the three step equation for the nitration of benzene
- 1: HNO3 + H2SO4 —> NO2+ + HSO4- + H2O
- 2: Benzene + NO2+ —> Nitrobenzene + H+
- 3: H+ + HSO4- —> H2SO4
Describe the halogenation of benzene
Benzene + Br2 —> bromobenzene + HBr
Room temp / pressure, halogens only react with benzene in the presence of a halogen carrier catalyst - AlX3 or FeX3 where X is the reacting halogen, which can be generated in situ (in the reaction vessel) from the metal and halogen.
Give the three step equation for the halogenation of benzene
- 1: Br2 + FeBr3 —> FeBr4- + Br+
- 2: Benzene + Br+ —> bromobenzene + H+
The bromonium ion receives a pair of electrons from the benzene ring (forming a dative covalent bond). The organic intermediate is unstable and breaks down to form bromobenzene and H+.
- 3: H+ + FeBr4- —> FeBr3 + HBr
Describe the alkylation of benzene
Benzene reacts with a haloalkane, forming an <alkyl>benzene and hydrogen halide in the presence of a halogen carrier catalyst.</alkyl>
Describe the acylation of benzene
When benzene reacts with an acyl chloride in the presence of AlCl3 halogen carrier catalyst, an aromatic ketone (phenyl<ketone>) is formed with HCl.</ketone>
Describe how alkenes decolourise bromine
Electrophilic addition.
The pi bond in the allele has localised electrons above and below the plane of C atoms, producing an area of high electron density.
Localised electrons in the pi bond induce a dipole in the non-polar Br2, making one Br partially positive and the other partially negative.
The positive Br enables Br2 to act like an electrophile.
Explain how benzene reacts with Br2
Doesn’t react with Br2 without a halogen carrier because it has delocalised pi electrons spread above and below the plane of C atoms.
The electron density around any 2 C atoms in the ring is less than in a C=C bond, so when a non-polar molecule approaches benzene, there is insufficient pi-electron density to polarise Br2 (allowing it to act like an electrophile), preventing a reaction.
What is a phenol?
A compound with an -OH group directly bonded to an aromatic ring.
Compounds with an -OH bonded to an alkyl side chain rather than the actual ring are alcohols not phenols.
Compare the solubility of phenols and alcohols
Phenol is less soluble in water than alcohols due to the non-polar benzene ring.
Is phenol acidic or alkaline and why?
When dissolved in water, phenol is a weak acid, as it dissociates into a phenoxide ion and a H+.
This means phenol can undergo neutralisation reactions with strong bases only.
Compare the acidity of carboxylic acids, alcohols and phenols
Alcohols < Phenols < Carboxylic acids
Alcohols don’t react with strong or weak bases.
Phenols react with strong bases only.
Carboxylic acids react with both strong and weak bases
What type of reaction does phenol undergo?
Electrophilic substitution, under milder conditions than benzene
Describe the bromination of phenols
phenol + 3Br2 —> 2,4,6-tribromophenol + 3HBr
Decolourises orange bromine water, forming a white precipitate of organic product. No halogen carrier required, under room temp and pressure
Describe the nitration of phenols
phenol + HNO3 —> 2/4-nitrophenol + H2O
Reacts with dilute HNO3 at room temperature
Explain the reactivity of phenols
Phenol is more reactive than benzene - it is nitrated with dilute HNO3 instead of concentrated HNO3 + H2SO4 like benzene.
This is because in phenol a lone pair of electrons is donated from the oxygen p orbital of the -OH group to the pi system of the benzene ring.
The increased electron density of the ring attracts electrophiles more strongly than in benzene.
The aromatic ring in phenol is therefore more susceptible to attack from electrophiles
Explain how activating groups work using an example
Br2 requires a halogen carrier to react with benzene, but reacts rapidly with phenylamine as NH2 is an activating group.
The -NH2 activates the ring as the aromatic ring reacts more readily with electrophiles.
Activating groups direct the 2nd substitutes to positions 2 or 4.
Explain how deactivating groups work using an example
Nitrobenzene reacts more slowly with Br2 than benzene, needing a halogen carrier and high temperature.
NO2 deactivates the aromatic ring as it reacts less readily with electrophiles.
Deactivating groups direct the second substituent to positions 3 on the ring.
What is the difference between an aldehyde and a ketone?
Aldehydes have the carbonyl functional group at the end of the carbon chain, ketones have the carbonyl functional group joined to 2 carbon atoms.
Describe the oxidation of ketones and aldehydes.
Aldehydes can be oxidised to carboxylic acids when refluxed with acidified dichromate ions (K2Cr2O7 / H+).
<aldehyde> + [O] --> <carboxylic>
Ketones do not undergo oxidation reactions
</carboxylic></aldehyde>
What is the type of reaction that a carbonyl double bond undergoes?
Nucleophilic addition
Why does the carbonyl double bond react differently to the alkene double bond?
C=O is polar but C=C is not. This means carbonyl compounds react with some nucleophiles, which are attracted to and attack the partially positive C atom, resulting in addition to the C=O bond.
What are the two compounds that a carbonyl functional group can react with?
- NaBH4
- HCN
Give the reaction and conditions when a carbonyl reacts with NaBH4
NaBH4 is used as a reducing agent to reduce carbonyl compounds to alcohols in warm, aqueous solution.
<carbonyl> + 2[H] --> <alcohol>
</alcohol></carbonyl>
Describe the mechanism for the reaction between a carbonyl compound and NaBH4
- NaBH4 contains the hydride :H- ion which acts as the nucleophile.
- Lone pair from :H- is attracted and donated to the partially positive C atom in the C=O bond, forming a dative bond.
- Pi bond in C=O breaks by heterolytic fission, forming a negatively charged intermediate.
- O atom of the intermediate donates a lone pair to a H atom in H2O. The intermediate has been protonated to form an alcohol
Give the reaction and conditions for the reaction between a carbonyl and HCN
HCN adds across the C=O bond of carbonyl compounds.
HCN is extremely poisonous and can’t be used safely in a lab, so NaCN and H2SO4 are used to provide it.
<carbonyl> + HCN --> <hydroxynitrile>
</hydroxynitrile></carbonyl>
Describe the mechanism for the reaction between a carbonyl and HCN
- Lone pair from :CN- is attracted and donated to the partially positive C atom in the C=O bond, forming a dative covalent bond.
- Pi bond in C=O breaks by heterolytic fission, forming a negatively charged intermediate.
- Intermediate is protonated by donating a lone pair of electrons to a H+, forming a hydroxynitrile.
How would you test for the presence of a carbonyl?
Add 2,4 DNP (dinitrophenylhydrazine). Positive test forms a yellow/orange precipitate
Describe the test distinguishing between aldehydes and ketones
Add Tollens’ reagent (AgNO3 in aqueous ammonia).
In the presence of aldehydes only, a silver mirror is produced.
The Ag+ ions in Tollens’ reagent act as an oxidising agent in the presence of ammonia, reducing Ag+ to Ag and oxidising the aldehyde to a carboxylic acid.
Ag+ + e- –> Ag(s) (silver mirror)
<aldehyde> + [O] --> <carboxylic>
</carboxylic></aldehyde>
Describe the solubility of carboxylic acids
The C=O and O-H bonds in carboxylic acids are polar and can form hydrogen bonds with water molecules.
Acids up to 4 carbons long are soluble in water. As the chain length increases, solubility decreases as the non-polar chain has a greater effect on polarity.
Dicarboxylic acids are solid at RTP and dissolve readily in water.
What is a carboxylate?
The organic ion produced when a carboxylic acid dissociates.
RCOOH <=> RCOO- + H+
Where RCOO- is the carboxylate ion.
Give the generic reaction for a carboxylic acid with Mg
RCOOH + Mg –> (RCOO-)2Mg2+ + H2
Give the generic reaction for a carboxylic acid with CaO
RCOOH + CaO –> (RCOO-)2Ca2+ + H2O
Give the generic reaction for a carboxylic acid with NaOH
RCOOH + NaOH –> RCOO-Na+ + H2O
Give the generic reaction for a carboxylic acid with Na2CO3
2RCOOH + Na2CO3 –> 2RCOO-Na+ + H2O + CO2
What is a carboxylic acid derivative?
A compound that can be hydrolysed to form the parent carboxylic acid.
Always contain an acyl group (R-C=O(X))
Give some examples of carboxylic acid derivatives
- Esters
- Acyl chlorides
- Acid anhydrides
- Amides
Describe how esters are formed, and what their functional group is?
Formed by the esterification reaction of an alcohol with a carboxylic acid.
Uses warm conditions and a concentrated H2SO4 catalyst, forming the ester and water.
Esters have the form: R-C(=O)-O-R
How are esters named.
Named by the alkyl group (from the alcohol), followed by the carboxylic acid with -oate instead of -oic acid.
e.g
propanol + ethanoic acid –> propyl ethanoate
What is hydrolysis?
The chemical breakdown of a compound in the presence of water
Describe ester acid hydrolysis
The reverse of esterification (so a reversible reaction).
Ester is refluxed with a dilute aqueous acid catalyst, forming the carboxylic acid and alcohol.
Describe ester alkaline hydrolysis
Irreversible and also known as saponification.
Ester is heated with aqueous OH- under reflux, forming a carboxylate ion and an alcohol.
A salt using the metal from the alkali and the carboxylate ion is then formed.
What is an acyl chloride and how are they named?
Have the form R-C(=O)-Cl.
Named from the parent carboxylic acid with -oic acid replaced by -oyl chloride.
e.g CH3COOH (ethanoic acid) –> CH3COCl (ethanoyl chloride)
Give the reaction for the preparation of an acyl chloride
RCOOH + SOCl2 –> RCOCl + SO2 + HCl
Describe the reactivity of acyl chlorides
They are very reactive and useful in organic synthesis as they can be easily converted to other carboxylic acid derivatives with good yields.
They react with nucleophiles by losing Cl- while retaining the C=O bond.
Describe the reactions of phenol with acyl chlorides, acid anhydrides and carboxylic acids.
Acyl chlorides and acid anhydrides are reactive enough to react with phenol, producing a phenyl ester and hydrogen chloride without an acid catalyst.
Carboxylic acids are not reactive enough to react with phenol at all.
Describe the reaction of acyl chlorides with water
Forms a carboxylic acid and hydrogen chloride in a violent reaction
Describe the reaction of acyl chlorides with ammonia and amines
Ammonia / amines act as nucleophiles by donating the lone pair on the N atom, forming amides.
The reaction with ammonia forms a primary amide:
RCOCl + 2NH3 –> RCONH2 + NH4Cl
The reaction with primary amides forms secondary amides:
RCOCl + 2CH3NH2 –> RCONHCH3 + CH3NH3+Cl-
How are acid anhydrides formed and how are they named?
Formed by the removal of water from 2 carboxylic acid molecules.
Named after the parent carboxylic acid molecule.
e.g:
2CH3COOH –> (CH3CO)2O + H2O
2ethanoic acid –> ethanoic anhydride + water
Describe the reactivity of acid anhydrides
They react in a similar way to acyl chlorides with alcohols, phenols, water, ammonia and amines, but are less reactive.
Describe the reaction of ethanoic anhydride with phenol.
Esterification reaction.
2(CH3CO)2O + C6H5OH –> CH3COOC6H5 + CH3COOH
ethanoic anhydride + phenol –> phenyl ethanoate + ethanoic acid
What are amines and what are the different types?
Organic compounds derived from NH3, where at least one H from ammonia has been replaced by a carbon chain or ring.
Primary amines have the N attached to 1 C atom, secondary amines have the N attached to 2 C atoms, tertiary amines have the N attached to 3 C atoms
Describe how you would name primary amines with examples
Primary amines on carbon 1 have the suffix -amine after the alkyl chain, e.g ethylamine.
Primary amines on any other carbon have the prefix amino- with its number, e.g 2-aminobutane
Describe how you would name secondary and tertiary amines with examples
Named by listing the groups attached to the N atom.
E.g (CH3)2NH = dimethylamine
CH3N(CH2CH3)CH2CH2CH3 = N-ethyl-N-methylpropylamine
Do amines react as acids or bases and why?
Behave as bases as the lone pair on the N atom can accept a proton.
This means they can neutralise acids to make ammonium salts, e.g:
CH3CH2NH2 + HCl —> CH3CH2NH3+Cl-
Give the conditions and reagents for the preparation of a primary amine
Excess NH3 is added to a haloalkane (excess as it reduces further substitution of the amine group to form secondary and tertiary amines).
Ethanol is used as a solvent to prevent substitution of the haloalkane with water (forming an alcohol).
Alkali (NaOH) is added to generate the amine from the ammonium salt formed.
Describe the mechanism from the preparation of a primary amine from a haloalkane
Ammonia has a lone pair on the N atom, allowing it to react as a nucleophile in a substitution reaction with a haloalkane, forming an ammonium salt. Alkali is then added to generate the amine from the ammonium salt.
Give an example equation for the preparation of a primary amine
CH3CH2Cl + NH3 —> CH3CH2NH3+Cl-
CH3CH2NH3+Cl- + NaOH —> CH3CH2NH2 + NaCl + H2O
Describe the preparation of secondary and tertiary amines with example equations.
Primary/secondary amines can be reacted again with haloalkanes to form secondary and tertiary amines.
CH3CH2Cl + CH3CH2NH2 —> (CH3CH2)2NH2+Cl- + NaOH —> (CH3CH2)2NH + NaCl + H2O
(CH3CH2)2NH + CH3CH2Cl —> (CH3CH2)3NH+Cl- + NaOH —> (CH3CH2)3N + NaCl + H2O
Describe how aromatic amines are prepared with example equations
Made by the reduction of nitrobenzene by heating under reflux with tin / concentrated HCl catalyst. Then excess NaOH is added to form the amine from the ammonium salt.
Nitrobenzene + 6[H] —> Phenylamine + 2H2O
What is an amino acid?
An organic compound containing both NH2 and COOH
How many amino acids occur naturally in the body and how are they classified?
There are 20 in the body, all of which are alpha amino acids.
Alpha amino acids have the amine group attached to the alpha carbon atom (the second C atom next to the carboxyl group).
What is the general formula of an alpha amino acid?
H2N-C(R)H-COOH
Describe the different types of reaction an amino acid can undergo
- Amine group is basic and so can react with acids to from salts (-NH2 turns into -NH3+)
- Carboxylic acid group reacts with alkalis to form a salt and water (e.g - COOH -> -COO-Na+)
- The carboxylic acid group is esterified by heating with an alcohol and H2SO4
What is an amide?
The products of reactions of acyl chlorides with ammonia / amines.
Have the general formula R-C(=O)NH-R
What is optical isomerism and what are optical isomers known as?
What types of organic molecules have optical isomers?
Non-superimposable mirror images.
Known as enantiomers.
Found in molecules containing a chiral centre (a C atom attached to 4 different groups).
How would you work out the number of enantiomers for any compound?
There are 2 pairs of enantiomers for each chiral centre, so:
Number of enantiomers = 2^(number of chiral centres)
What is condensation polymerisation?
The joining of monomers with the loss of a small molecule per bond formed, usually H2O or HCl.
Requires two different functional groups to occur.
What are polyesters and what can they be formed from?
Polymers where the monomers are formed by ester linkages in a long chain.
Either made from 1 monomer with both a carboxylic acid and alcohol group, or a dicarboxylic acid with a diol.
What are polyamides and what are they formed from?
Polymers made from monomers joined by amide linkages (-C=O-NH-).
Either made by one monomer with a carboxylic acid and an amine group, or a dicarboxylic acid and a diamine.
What are the 2 ways condensation polymers can be hydrolysed?
- Hot alkali: A salt is formed between the newly formed carboxylate ion (COO-) from the monomer and the alkali
-Hot acid: Forms the monomer with NH3+ if an amine group is present
What functional group is used for carbon-carbon bond formation synthesis and why is it important?
Uses the nitrile group -CN.
Important for lengthening carbon chains, adding side chains to benzene rings and introducing new functional groups
Describe how nitrile compounds can be formed
Formed by reacting haloalkanes with NaCN or KCN in ethanol by a nucleophilic substitution reaction, forming a potassium/sodium halide.
Describe the curly arrow mechanism for the reaction between a haloalkane and CN-
- Arrow from lone pair of the fully negative CN- to the partially positive carbon
- Arrow from the carbon-halide bond to the partially negative halide
- Product formed
Describe how hydroxynitrile compounds can be formed
Carbonyls (aldehydes/ketones) react with hydrogen cyanide in a nucleophilic addition reaction, forming a hydroxynitrile. Hydrogen cyanide is too poisonous to use so a mixture of sodium cyanide and sulfuric acid is used.
Describe the curly arrow mechanism for the reaction between an aldehyde/ketone and NaCN with H2SO4
- Arrow from lone pair of the fully negative CN- to the partially positive carbon
- Arrow from one of the C=O bonds to the partially negative oxygen
- Intermediate with CN attached and a C-O(-) bond
- Arrow from lone pair of the O- to the partially positive hydrogen on a water molecule
- Arrow from the O-H bond of water to the partially negative oxygen of water
- Product and OH- formed
Describe the reduction reaction of nitriles
Nitriles can be reduced to amines by reacting with hydrogen in the presence of a nickel catalyst.
e.g
CH3CH2CN + 2H2 –> CH3CH2CH2NH2
Describe the hydrolysis reaction of nitriles
Nitriles undergo hydrolysis to form carboxylic acids by heating with dilute aqueous acid.
e.g
CH3CH2CH2CN + 2H2O + HCl –> CH3CH2CH2COOH + NH4Cl
What are the two ways to form carbon-carbon bonds to benzene rings?
- Acylation
- Alkylation
Describe the alkylation reaction of benzene
Transfers an alkyl group from a haloalkane to a benzene ring in the presence of a Friedel-Crafts catalyst (halogen carrier), forming a hydrogen halide
Describe the acylation of benzene
Benzene reacts with an acyl chloride in the presence of an AlCl3 catalyst, forming a ketone and HCl.
What is filtration under reduced pressure used for?
Used to separate a solid product from a solvent/ liquid mixture of products.
What is the equipment needed for filtration under reduced pressure?
- Buchner flask
- Buchner funnel
- Pressure tubing
- Filter paper
- Access to filter / vacuum pump
Describe the process for filtration under reduced pressure
- Set up Buchner filter by connecting pressure tubing to flask and filter vacuum pump
- Fit Buchner funnel to the flask with a bung for a tight fit and switch on the tap / pump
- Place filter paper in the funnel and wet it with the solvent.
- Pour the reaction mixture in and let filter
- Rinse beaker with the solvent and leave filter on to dry so crystals form in the top
What is recrystallisation used for?
Used to purify a solid product that has impurities with different solubilities in the chosen solvent
Describe the process of recrystallisation
- Dissolve the solid in the minimum amount of hot solvent
- Filter under reduced pressure
- Let the solution cool, so crystals form, and filter under reduced pressure again
- Wash with the minimum amount of cold solvent and let dry
What is melting point determination used for and how?
Used to identify whether a solid compound is pure.
If the compound melts over a range and at a lower temperature than the known melting point, it is impure.
How would you prepare a sample for melting point determination?
Hold the end of a capillary tube in a bunsen burner to close it, and push the open end into the solid sample until there is about 3mm inside the tube.
What are the two methods that can be used to determine melting point of a compound?
- Electrically-heated melting point apparatus
- Oil bath / Thiele tube
Describe how you would use electrically-heated melting point apparatus to determine the melting point of a sample
- Place capillary tube in the sample hole and a 0-300 degree thermometer in the thermometer hole
- Use rapid heat setting to heat the sample until you see it melt, then record melting point
- Allow to cool and test again, heating slowly as the melting point is approached, to get a more accurate value
Describe how you would use an oil bath / Thiele tube to determine the melting point of a sample
- Set up Thiele tube / oil bath and attach sample capillary tube to a thermometer with a rubber band
- Insert thermometer through a hole in the cork in a Thiele tube / clamp thermometer in an oil bath so the end of the thermometer and capillary tube dip into the oil
- Heat arm of Thiele tube / oil bath with a micro burner and record the temperature sample melts at
- Remove heat once it begins to melt so it heats slowly as the melting point is approached, making the value more accurate
What is chromatography used for?
Used to separate individual components from a mixture of substances
Give some applications of chromatography
The analysis of drugs, plastic and flavourings, and has applications in forensic science.
Describe the two phases in all forms of chromatography
- Stationary phase - doesn’t move, usually a solid or a liquid layer on a solid
- Mobile phase - does move, usually a liquid or a gas
What is thin layer chromatography (TLC) used for and describe the equipment needed?
A quick and inexpensive technique indicating how many components are in a mixture.
Uses a TLC plate (usually a plastic sheet or glass) coated with a thin layer of a solid adsorbent substance, usually silica.
Describe the mobile and stationary phases in thin layer chromatography (TLC)
The mobile phase is the liquid solvent.
The adsorbent (silica) is the stationary phase - different components in the mixture have different affinities with the adsorbent, and bind to the surface with different strengths.
Separation is achieved by the relative adsorptions of substances with the stationary phase.
What is adsorption?
The process by which solid silica (in TLC) holds the different components of the mixture to its surface.
Describe the method for TLC
- Draw a pencil line about 1cm from the end of the TLC plate, and spot a small sample onto it.
- Prepare a chromatography tank (small beaker with a watch glass on top). Add 0.5cm of solvent
- Place TLC plate in the beaker so the solvent doesn’t touch the base line and leave it so the components separate.
- Take TLC out and mark the solvent front with a pencil - allow to dry
- Draw around visible spots with a pencil, or use UV / a locating agent like iodine to show invisible spots
How are TLCs quantitatively analysed?
TLCs are analysed by calculating the retention factor (Rf) values for each component and comparing with known Rf values for the given substances.
What is the equation for Rf values?
Rf = (distance travelled by component) / (distance travelled by solvent front)
What is gas chromatography used for?
Used for separating and identifying volatile organic compounds present in a mixture.
Describe the stationary and mobile phases in gas chromatography
Stationary phase is a high boiling point liquid adsorbed on an inert solid support.
Mobile phase is an inert carrier gas, e.g helium or neon
Describe how gas chromatography works
- Small amount of volatile mixture is inserted into has chromatogram. Mobile gas carries components in a sample through capillary column which contains the liquid stationary phase adsorbed onto the solid support.
- Components slow down as they interact with stationary phase inside column. More soluble the component is in the liquid stationary phase = moves slower in capillary column.
- Components separated based on their solubility in the stationary phase liquid. Compounds reach the detector at different times based on their interaction with the stationary phase.
What is retention time?
In gas chromatography, it is the time taken for each component to travel through the column. Compound retained in the column for the shortest time has the lowest retention time and is detected first.
What information is found on a gas chromatogram?
Each component is detected as a peak on the gas chromatogram. Has 2 pieces of information:
- Retention time - used to identify components present in sample by comparing to known values.
- Peak integrations - used to determine components’ concentrations
How would you determine the concentration of a component in a sample using gas chromatography?
- Prepare standard solutions of known concentrations of the target compound
- Obtain gas chromatograms for each standard solution
- Plot a calibration curve of peak area against concentration. This is known as external calibration
- Obtain gas chromatogram of the target compound under the same conditions
- Use calibration curve to find concentration of the original sample
Describe the test used to identify an alkene
Add bromine water drop-wise, decolourises bromine water, orange to colourless.
Describe the test used to identify a haloalkane
Add AgNO3 and ethanol, warm to 50 degrees in a water bath.
Chloroalkane = white precipitate
Bromoalkane = cream precipitate
Iodoalkane = yellow precipitate
Describe the test used to identify a carbonyl compound
Add 2,4 dinitrophenylhydrazine (DNP), forms an orange precipitate
Describe the test used to identify an aldehdye
Warm with Tollens’ reagent, forms a silver mirror
Describe the test used to identify primary/secondary alcohols and aldehydes
Add acidified potassium dichromate and warm in a water bath. Colour changes from orange to green.
Describe the test used to identify carboxylic acids
Add aqueous sodium carbonate, observes effervescence
Describe in general terms how nuclear magnetic resonance (NMR) spectroscopy works
Uses a combination of a very strong magnetic field and radio frequency radiation. With the right combination, the nuclei of some atoms absorb the radiation. The energy for the absorption can be measured and recorded as an NMR spectrum
What is nuclear spin and why is it important for NMR spectroscopy?
All atoms’ nuclei have a nuclear spin, which is significant if there are an odd number of nucleons (protons + neutrons). In organic chemistry, this is why the 1H and 13C isotopes are important for analysis (as 12C isotopes have a weak spin). NMR can detect isotopes of other elements with odd nucleons, but 1H and 13C are the most common
What is resonance?
Electrons have 2 different spin states, and so do nuclei, each with different energies. With the right combination of a strong magnetic field and radio frequency radiation, the nucleus can absorb energy and flip rapidly between spin states - this is resonance.
Explain the structure of an NMR spectrometer
Uses a very strong super-conducting electromagnet cooled to 4K by liquid helium.
This is because radio frequency radiation has little energy - the frequency needed for resonance is proportional to magnetic field strength and so can only be detected in strong and uniform magnetic fields.
What is chemical shift in NMR spectroscopy?
All atoms have electrons surrounding the nucleus, which shifts the energy / radio frequency needed for NMR to take place.
Frequency shift is measured on a scale called chemical shift, measured in ppm (parts per million)
What is the standard reference chemical for chemical shifts in NMR spectroscopy?
Tetramethylsilane (TMS), (CH3)4Si. It has 0ppm chemical shift, and is the compound against which all chemical shifts are measured.
What is chemical shift determined by?
The chemical environment of that atom, which produces absorption peaks at different chemical shifts (chemical environment changes the energy / frequency needed for NMR).
How is an NMR spectrum run?
To run the spectrum, the sample is dissolved in a solvent and placed in a narrow NMR sample tube with a small amount of TMS.
Inside the spectrometer, it is spun to even out imperfections in the magnetic field, and the machine is zeroed against the TMS standard.
Sample is given a pulse of radiation with a range of radio frequencies - any absorptions of energy from the resonance are detected and plotted on a spectrum.
The sample can then be recovered by evaporating the solvent.
What solvents are used in NMR and why?
Most solvents contain 13C and 1H, which produce a signal in 13C and 1H spectra.
Instead, deuterated solvents are used (all 1H atoms are replaced by D - deuterium - 2H), as deuterium doesn’t produce a signal in NMR frequencies.
Deuterated trichloromethane (CDCl3) is a common solvent, but still produces a peak in a 13C spectrum.
What information does a 13C NMR spectrum give you?
- Number of different carbon environments from the number of peaks.
- Types of carbon environment present from the chemical shift
What are carbon environments and how do they relate to 13C NMR spectra?
Carbon environments are determined by the position of the carbon in the molecule.
Carbon atoms bonded to different groups have different environments so will absorb at different chemical shifts.
If 2 carbon atoms are positioned symmetrically within a molecule, they are equivalent and have the same chemical environment, so will absorb radiation at the same chemical shift and contribute to the same peak.
How many carbon environments are there in:
- CH3CH2CHO
- CH3COCH3
- 3 carbon environments so 3 peaks on the spectrum
- 2 carbon environments so 2 peaks on the spectrum
What information does a 1H (proton) NMR spectrum tell you?
- Number of different hydrogen environments from the number of peaks
- Types of hydrogen environments from the chemical shifts
- Relative numbers of each type of hydrogen from the relative peak areas (peak integration)
- Number of non-equivalent protons adjacent to a given proton from the spin-spin splitting pattern
How do proton environments affect 1H NMR spectra?
If 2 or more protons are equivalent, they absorb at the same chemical shift, increasing the size of the peak.
Protons in different environments absorb at different chemical shifts
Describe how splitting patterns in proton NMR spectra work
A proton NMR peak can be split into sub-peaks due to the proton’s spin interacting with the spin states of adjacent non-equivalent protons. This is spin-spin coupling / splitting.
It follows the n+1 rule - the number of sub-peaks is 1 + the number of adjacent non-equivalent protons.
The peaks are named singlet, doublet, triplet, quartet.
What are the two bond types difficult to assign a peak in proton NMR and why?
NH and OH protons may be involved in hydrogen bonding in solution, so their NMR peaks are often broad and of variable chemical shift.
They are also not usually involved in spin-spin coupling, so it is difficult to assign OH and NH protons.
What is proton exchange in proton NMR spectroscopy and how do we do it?
A technique used to identify OH and NH protons on NMR spectra.
Proton NMR spectrum is run as normal. A small volume of deuterium oxide, D2O is added, the mixture is shaken and a second spectrum is run.
How does proton exchange work to identify OH and NH protons in NMR spectra?
Deuterium exchanges and replaces the OH and NH protons in the sample with deuterium atoms.
e.g CH3OH + D2O –> CH3OD + HOD
As deuterium doesn’t absorb in the proton NMR chemical shift range, the OH and NH peaks disappear on the second spectrum, and so can be identified on the first
