Topic 17: Further Organic Flashcards
What is optical isomerism?
Stereoisomerism has 2 types. Ya sabes geometric, now learn optical.
Optical isomers: non-superimposable mirror images of each other w asymmetry. This is chirality.
Enantiomers: chiral molecules w C as the chiral centre.
Chirality arises if a molecule contains a tetrahedral C atom w 4 DIFF groups attached. If 2 groups are the same, there’s no chirality bc one can be superimposed on the other.
What is the difference between an unpolarised wave and a plane polarised wave?
Unpolarised wave: oscillations may occur in any plane or direction.
Plane polarised wave: oscillations only occur in 1 plane/direction.
Some materials can absorb all of the oscillations except those in a single plane, and so convert unpolarised light into plane polarised light.
Describe and explain polarimetry and how a polarimeter works
Polarimetry: using a polarimeter to measure the amount of optical activity. A monochromatic light passes thru a polarising filter called the polariser. This converts unpolarised light into vertically plane polarised light.
Plane polarised light passes thru a sample tube w some of the substance in solution. If the substance is optically active (bc it is an enantiomer), then the plane of polarisation rotates. Clockwise rotation means the substance is dextrorotary. Anticlockwise rotation= laevorotatory.
The second polarising filter (the analyser) is rotated to a position where max light intensity can be seen. The angle of rotation is measured. It is + if the rotation is clockwise and negative if the rotation is anticlockwise.
Describe physical and chemical properties of two enantiomers.
2 enantiomers= identical phys properties except, they rotate plane polarised light by equal angles, but opp directions. They’re thus optically active. So if rotation for 1 enantiomer is +60°, rotation for the other is -60°.
2 enantiomers have identical chem properties w 1 exception: the way in which they react with enantiomers of other substances. This property may be diff for each enantiomer.
When hay equal quantities of 2 enantiomers the mixture is racemic. No hay optical activity bc the anticlock and clockwise rotations on plane polarised light cancel out.
Draw the SN2 mechanism for nucleophilic substitution of 3-fluro-3-bromoethane and explain the product formed.
This is an enantiomer. Only 1 product isomer is formed. If you had 1 enantiomer to begin w, you get the opp one in the product. if reactant=dextrorotary, product= laevorotary.
By measuring optical activity of the og haloalkanes and the alcohol formed, we can show if the reaction occurred by SN2.
Draw the SN1 mechanism for 3-fluro-3-bromoethane
The og haloalkane has a tetrahedral shape, but the product of step 1 is a planar carbocation.
This means that in step 2 hay an equal chance that the attacking HO:- can approach from the left OR right. Therefore hay 2 enantiomer products present in equal numbers, forming racemic mixture. The product will have no optical activity.
Describe polar bonding in carbonyls
The electron density in the pi bond alkenes are evenly distributed across both carbon atoms.
In carbonyls however, the C=O bond is polar bc of differing electronegativities of carbon and oxygen. The electron density is greater near the oxygen rather than the carbon atom.
Describe bpt of carbonyls
Aldehydes and ketones have higher intermolecular forces than alkanes.
They contain the polar C=O group so they have permanent dipole-dipole attractions. No H bonding as all their H atoms are joined to C atoms.
As with alkanes and alcohols, boiling temps increase w increasing chain length (more Ldn forces). At room temp methanal is a gas. The other carbonyl compounds are liquids.
Describe solubility of carbonyls
Lower aldehydes and ketones are soluble in water because they can form H bonds with water molecules.
Solubility of aldehydes and ketones decreases with increasing chain length as the hydrocarbon part of the molecule increases and is not polar.
How can aldehydes and ketones can be reduced to alcohols? Use propanal and butan-2-one as examples
Both aldehydes and ketones can be reduced to alcohols by LiAlH4. Both the carbonyl compound and reducing agent are dissolved in dry ether.
The reducing agent is represented by H.
CH3CH2CHO + 2[H] –> CH3CH2CH2OH
CH3COCH2CH3 + 2[H] –> CH3CH(OH)CH2CH3
Describe how to test for aldehydes and ketones using Acidified potassium dichromate.
Acidified potassium dichromate heated under reflux.
The colour goes from orange Cr2O72- to green Cr3+ Reduced from Cr(+6) to Cr(+3)
Describe how to distinguish between aldehydes and ketones using Fehlings/Benedict’s solution.
Reagents: copper sulphate solution & NaOH solution, warm gently
Aldehydes form a brick red ppt, ketones don’t form a brick red ppt. Aldehydes reduce Cu2+ to Cu(I) oxide.
Because the solution is alkaline, the aldehyde itself is oxidised to a salt of the corresponding carboxylic acid.
RCHO + 2Cu 2+ + 5OH- -> RCOO- + Cu2O + 3H2O
Describe how to distinguish between aldehydes and ketones using Tollen’s reagent
This is an oxidation reaction, Tollen’s is the oxidising agent
Reagents: silver nitrate solution, few drops of NaOH (aq) and a little ammonia solution. Warm gently.
Result: Aldehydes: silver mirror Ketones: no silver mirror
Aldehydes reduce the diamine silver(I) ion to metallic silver. Because the solution is alkaline, the aldehyde is oxidised to a salt of the carboxylic acid.
Describe how to distinguish between aldehydes and ketones using the iodoform reaction
Reagents: solution of iodine in potassium iodide & NaOH solution at room temp.
Positive result = yellow ppt and antiseptic smell
This shows the presence of a C=O next to a methyl group (we call it a methyl carbonyl) OR an –OH next to a terminal CH3 group
Describe how to detect a carbonyl using 2,4-DNP (Brady’s reagent)
Reagents: 2,4 DNPH, warm. Result: yellow/orange ppt.
The reaction is a condensation reaction and water is formed. (nucleophilic addition-elimination reaction).
To determine WHICH carbonyl was present: the ppt is filtered off, recrystallised and the mpt found. This can be compared to known mpts of 2,4-DNPH derivative to identify the original carbonyl.
Describe reactivity of aldehydes vs ketones
Aldehydes are generally more reactive than ketones bc the presence of the two alkyl groups hinders nucleophilic attack and reduces the partial + charge on the carbon atom of the carbonyl group.
What is recrystallisation?
Recrystallisation purifies a solid substance. When a compound is isolated from a reaction mixture it contains impurities which are removed by dissolving the substance in a suitable hot solvent and leaving to cool.
As the solution cools the solubility of the compound decreases and purer crystals are formed. Recrystallisation may be repeated several times.
Impurities in a substance will affect its mpt and make it difficult to identify the compound isolated from a reaction.
Why is HCN so dangerous?
Highly toxic because it inhibits a mitochondrial enzyme for resp.
It is hard to handle safely because it’s so volatile and flammable.
A safer alternative is KCN which is solid at room temp and it’s easier to handle. An acidified solution contains both H+ and CN- ions
How do carbonyls react w hydrogen cyanide?
The reaction is carried out in aq alkaline solution of sodium cyanide. A hydroxynitrile forms.
A H atom attaches to the O of the carbonyl group and a CN group attaches to the carbon atom of the carbonyl group.
Draw the mechanism for the reaction : nucleophilic addition of KCN to ethanal
Describe the melting and boiling pts of carboxylic acids
High bpts due to H bonding (higher than alkanes).
Boiling temperatures increase with increasing molar mass because of more London forces.
Describe solubility of the carboxylic acids
Small carboxylic acids are soluble in water. H bonds form between water and carboxylic acid molecules.
Solubility of the bigger acids decreases rapidly w size. Longer hydrocarbon part of the molecule increases and is not polar.
What kind of acids are carboxylic acids?
Carboxylic acids are weakly acidic bc of the slighlty + H in the COOH.
The - charge is delocalised across the carboxylate group resulting in a more stable ion. Delocalisation is the dotted line to the CO bonds which are both equivalent.
So carboxylic acids PARTIALLY dissociate in aq solution
How are carboxylic acids made weaker and stronger?
Electron donating groups like methyl groups push e towards the carboxylic acid carbon. This stabilises the OH bond and makes it harder to break. EDG make acids weaker, hay less dissociation so Ka is smaller
Electron withdrawing groups eg halogens or electroneg groups pull e away from the carboxylic acid carbon, weakening the OH bond, making it easier to break. EWG make acids stronger, hay mas dissociation, Ka is bigger
CH3COOH + H20 → CH3C00- + H3O+
State the 3 ways you can make carboxylic acids
1) strong oxidation of a primary alcohol w acidified k dichromate under reflux
2) further oxidation of an aldehyde
3) hydrolysis of a nitrile. Reagents: dilute HCl/H2SO4 heated under reflux
CH3CN + 2H2O + H+ → CH3COOH + NH4+
what are amines?
Amines: organic compounds based on ammonia, NH3 , in which one or more of the hydrogen atoms are replaced by alkyl or aryl groups.
Amines have a fishy or rotting smell
How do we name primary amines?
C2H5NH2 ethylamine
C4H9NH2 butylamine
C6H5NH2 phenylamine
How do you name secondary amines?
use dimethylamine and N-methylpropyl amine as an example
How do you name tertiary amines?
Tertiary: trialkyl prefix if chains same, e.g. trimethylamine
OR: N,N-alkylalkylalkylamine
e.g. N,N-ethylmethylpropylamine
Name the following:
What happens when other NH2 functional groups are present in the amine?
If other functional groups are present in the molecule, the presence of amine groups is denoted using the amino– prefix.
Describe general Amine/acid neutralisation reactions
Amines accept H+ from acids to form quarternary ammonium salts.
If the reaction is carried out in solution, the amine accepts a H+ from H3O+ to form an ionic salt and water.
draw the reaction for methylamine and nirtic acid
Draw the reaction for diethylamine and sulfuric acid
Describe amine reactions with water
Amines are soluble in water, but they also react with water (like NH3): amine + water → ammonium ion + OH-
How are amiDES formed?
Amides can be formed by reacting: A carboxylic acid with an amine OR An acyl chloride w conc ammonia
Acyl chlorides are more reactive so these are preferred in many reactions.
How do u form POLYAMIDES
A molecule with a carboxylic acid/acyl chloride at one end and an amine at the other can form an amide.
If your molecules have functional groups at BOTH ends, they can carry on reacting, forming big long chains of molecules joined through amide links:
diacid monomer + diamine monomer → poly (amide)
How are proteins hydrolysed?
Prolonged heating w conc HCl. Bc of the strongly acidic conditions all amino acids formed will have the NH2 groups protonated:
Describe the conditions required for nitration of benzene.
Conc HNO3 and conc H2SO4 (aka the nitrating mixture), 50°C. More N02 groups can be subbed if mix is hotter. To ensure mono substitution place the mix in an ice bath.
Nitrobenzene is used in dyes, pesticides pharmaceuticals. As benzene can’t induce a dipole the NO2+ electrophile is made by the nitrating mixture.
How is nitrobenzene used to make an aromatic amine?
Nitrobenzene + tin + conc HCl heated under reflux act as reducing agents.
Followed by NaOH to neutralise excess H+
Phenylamine is formed which is separated from the mix
Describe halogenation of benzene
Benzene can’t induce a dipole (too stable), so halogens dont react w it unless a halogen carrier catalyst, eg AlCl3 or FeBr3 is used. This polaries the halogen and forms an electrophile.
Describe alkylation of benzene
Alkylation is a type of Friedel-Crafts reaction. Used to sub any alkyl R grup onto benzene:
Describe acylation of benzene using an acyl chloride
Acylation substitutes a COR group onto the benzene ring. It is an electrophilic substitution reaction done under reflux.
Draw the mechanism for acylation of benzene w an acid anhydride
Does phenol react with bromine water?
Unlike benzene, phenol reacts w a solution of Br water at room temp sin catalyst
2,4,6-tribromophenol (white ppt) is formed. The electrophile is formed from the reaction of Br w water.
Describe the reaction of phenol with dilute and conc nitric acid
Phenol reacts w dilute HNO3 but benzene needs a nitrating mixture. This is bc one of the lone pairs on the oxygen atom in the OH group overlaps with the delocalised ring of e-, making phenol mucho mas reactive.
Describe reactions of phenol with sodium and sodium hydroxide
Bc it can act as a weak acid, phenol reacts w bases in neutralisation reactions.
Phenol + NaOH forms sodium phenoxide which is soluble in water. The reaction can be reversed by adding a strong acid.
Phenol + Na forms sodium phenoxide and hydrogen gas
What happens when an alcohol reacts with a carboxylic acid?
Alcohol + carboxylic acid –> ester + water. Heat under reflux w conc H2SO4 catalyst. The reaction is reversible and occurs very slowly.
Describe the reaction of acyl chloride + water
Describe the reaction of acyl chloride + alcohol
Use ethanoyl chloride and ethanol as an example
Describe the reaction of acyl chloride w an amine
N-substituted amide. Secondary amines react the same, but the product has 2 substituted alkyl groups
This reaction does not work w tertiary amines. W 3 Alkyl groups no hay H atoms to react with chlorine to form HCl.
Describe the reaction of acyl chloride with phenylamine
What is an acid anhydride?
It’s forms from a condensation reaction between 2 carboxylic acids
How can acid anhydride be used to make esters? Use ethanoic anhydride and methanol as an example
Acid anhydride + alcohol –> Ester + carboxylic acid
Describe hydrolysis of esters/ polyesters. Use methyl propanoate as an example
Ester + water OR dilute acid –> carboxylic acid + alcohol
Hydrolysis w pure H20 is v slow so never used. For acid, the ester is heated under reflux w dilute HCl/H2SO4. Reactions=reversible, hydrolysis incomplete.
Ester + dilute alkalai –> Na-oate + alcohol. Heat under reflux. Irreversible, hydrolysis complete. You get a sodium salt instead so products are easier to separate. Only add acid to convert the salt to a carb acid.
