Unit 4 Questions Flashcards
What are stereoisomers?
Isomers with the same structural formula but different orientations in space
What are two molecules in mirror image called?
Enantiomers
What are enantiomers?
Two molecules in mirror image
What is a chiral molecule?
A molecule that cannot superimpose
What does “optically active” mean?
They rotate the plane of polarised light
What is a racemic mixture?
An equimolar mixture of enantiomers that is optically inactive
Give three pieces of evidence against Kekule’s structure of benzene
- Benzene doesn’t decolourise bromine, therefore has no pi-bonds
- X-ray spectroscopy shows equal bond lengths, which means there can’t be a mixture of singles and doubles
- Hydrogenating cyclohexene to cyclohexane should release 360 kJmol-1 but we only get 208, so benzene is more stable
What is resonance energy?
The difference between the experimental delta H reaction for benzene and the delta A reaction according to Kekule’s structure.
Why is benzene more stable than Kekule’s structure?
Because of the electron delocalisation
Give a detailed explanation of the structure of benzene
A planar ring of carbon atoms bonded to one another and to their hydrogens by sigma (sp2) bonds. Each carbon has one unused p orbital with one electron which sit perpendicular to the plane of the ring with one lobe above and one below. Each p-orbital overlaps sideways with two neighbouring orbitals to form a single pi-bond that extends as a ring of charge above and below the plane.
A diagram would probably be good.
What kind of molecules does benzene attract?
Electrophiles because of the great electron density
What do the characteristic reactions of benzene all involve and why?
They all involve substitution because the delocalised ring of electrons makes benzene incredibly stable. Addition would destroy the ring, requiring more energy than it would take to break a C=C bond.
Nitration of benzene. Name the reagents, conditions, and type of reaction.
What happens if the temperature is too high?
Describe/draw the mechanism
Reagents: concentrated sulphuric acid (H2SO4) and concentrated HNO3
Conditions: reflux, 55 degrees C
If the temperature goes above 55, further substitution makes dinitrobenzene.
For the rest see diagram A
What are the two methods of halogenating benzene?
By electrophilic substitution or by free radical addition
Halogenating of benzene by free radical addition: conditions and skeletal equation diagram
UV light at a warm temperature
For the reaction see diagram B
Halogenation of benzene via electrophilic substitution: conditions and skeletal mechanism
Room temperature and a halogen carrier (Friedel-Crafts catalyst)
For reaction see diagram C
Alkylation of benzene: reagents and skeletal mechanism
The alkyl group must be within a halogenoalkane and needs a Friedel-Crafts catalyst
For mechanism see diagram D
Why does hydrolysis of benzene not work under normal conditions when the halogen is attached directly to the ring? What conditions does it need?
The C-Cl bond is too strong in chlorobenzene due to the delocalisation of the pi electron cloud of the ring with the p-orbital electrons (or lone pairs) of the chlorine atom. Furthermore, benzene is already electron rich and therefore not susceptible to nucleophilic attack.
Hydrolysis needs extreme conditions such as 200atm, 473K.
Forming alcohols from halogenoalkanes: reagents, conditions, reaction type.
Give the mechanism (displayed)
What kinds of alcohols does this not work well for and why?
Reagents: Aqueous NaOH solution
Conditions: warm, reflux
Type: nucleophilic substitution
Doesn’t work well for aromatic halides with the halide directly on the ring, as the C-Cl bond is less polar/stronger and therefore less susceptible to nucleophilic substitution
For the mechanism see diagram E
Forming alcohols from carbonyl compounds: give the two reducing agents used and their respective conditions and uses
NaBH4 aqueous at room temperature. Cannot directly reduce carboxylic acid to primary alcohol
LiAlH4 in dry ether at 0 degrees. Used when NaBH4 cannot be.
Formation of alcohols from alkenes: reagents, conditions, and reaction type. The mechanism’s pretty easy so I won’t ask for that.
Add water. Needs phosphoric (V) acid (H3PO4) as a catalyst.
350 degrees, 60 atm
Hydration reaction
Name the three types of compounds from which we can make alcohols
- Alkenes
- Halogenoalkanes
- Carbonyl compounds
Show the oxidisation process of primary alcohols with half ionic equations
See diagram F
What is produced by oxidising each type of alcohol?
Primary —> aldehyde —> carboxylic acid (reflux and excess oxidising agent)
Secondary —> ketone
Tertiary alcohols cannot be oxidised as there are no hydrogens on the hydroxy-bearing carbon
What is an alternative way of making aldehydes and ketones that doesn’t involve adding an oxidising agent to an alcohol?
Passing the alcohol over a copper catalyst at 500 degrees
Dehydration of alcohols: what do we make and what are the standard reagents and conditions?
Name two other reagents and conditions, if they have any
We make an alkene by losing a water molecule. Standard reagents are excess concentrated H2SO4 and 180 degrees
Other reagents: alumina (AlO3) and ethanol vapour at 350 degrees
Hot concentrated phosphoric (V) acid (H3PO4)
Describe what kind of reaction the halogenation of an alcohol is. What is the yield like? What can it be used for? What do secondary alcohols give and how do we see this?
Nucleophilic substitution
Slow, reversible, and low-yield
Can be used to test the stability of carbocations
Secondary alcohols make secondary halide compounds. These are insoluble and give some cloudiness over time.
Chlorination of an alcohol: give the conditions, standard reagents, two alternative reagents, and general equation
Anhydrous zinc chloride (catalyst) and HCl gas with heat
Alternatively, phosphorous (V) chloride, PCl5, or sulphur dichloride oxide, SOCl2
R-OH + HCl —> R-Cl + H2O
Bromination of an alcohol: give the conditions and general equation
Potassium bromide (KBr) and 50% H2SO4, warm
R-OH + KBr + H2SO4 —> R-Br + H2O + KHSO4
Iodination of an alcohol: give the reagents and conditions
Red phosphorus (P) and iodine (I2), warm
Give the reagents, conditions, products, likely yield, and equation of the reaction of an alcohol with acyl chloride
Anhydrous ethanoyl chloride, room temperature. Products are an ester.
Maximum yield possible as the reaction is not reversible
for equation see diagram G
What is an alternative name for the reaction of alcohol with acyl chloride?
Ethanoylation
What is an alternative name for the reaction between an alcohol and a carboxylic acid?
Esterification
Reaction of an alcohol with a carboxylic acid: reagents, conditions, product, and how to isolate that product. Give displayed mechanism as well (the 1-step one)
Concentrated H2SO4, heat
The product is an ester, and to attain it we add sodium hydrogen carbonate solution to neutralise the leftover acid, separate using a separating funnel, dry and distil ester layer.
For mechanism see diagram H
Give the detailed mechanism of the reaction between an alcohol and a carboxylic acid
There’s no way I’m typing that out. See diagram I
Give four physical characteristics of phenol
- Crystalline
- Colourless
- Oxidises easily to become pink
- Hydrogen bonding makes it slightly miscible in water
How does the acidity of phenol compare to that of ethanol? Is the remaining phenoxide ion stable?
Draw a skeletal mechanism of the delocalisation of phenol in water.
It is more acidic than ethanol in aqueous solution because the lone pair from the oxygen is displaced towards the benzene ring, effectively releasing the H+.
The remaining phenoxide ion is stable because the oxygen’s p-orbital joins the delocalised pi electron ring.
For the mechanism see diagram J
Give two methods for distinguishing alcohols from phenols
- Add excess bromine. Phenol forms a white ppt, alcohols don’t decolourise
- Add aqueous FeCl3. Phenol goes violet, alcohol remains orange.
Which undergoes electrophilic substitution better, phenol or benzene?
Phenol of course! That was easy
Give the two general steps and one observation of electrophilic substitution reactions in phenol. Give a skeletal mechanism of the first.
- All reactions start with adding NaOH to make sodium phenoxide (for mechanism see diagram K)
- The hydroxy group activates the delocalised ring, causing a build-up at the 2-, 4-, and 6- positions.
You would observe white misty fumes of hydrogen halide before dissolving into, say, hydrobromic acid.
What is the difference between the reaction of phenol with sodium carbonate and the reaction between an acid and sodium carbonate?
Acids produce CO2 effervescence, turning limewater cloudy.
Despite being weakly acidic, phenol does no such thing.
What does ethanoylation produce? Is this faster in aliphatic or aromatic compounds?
Ethanoylation produces an ester. This is much faster in aliphatics.
Give the two-step process of ethanoylation (makes an ester) of phenol, with equations
First, make a phenoxide ion by putting the ester in an alkaline solution (I’m gonna be brave and type this out)
C6H5OH + OH- —> C6H5O + H2O
The resulting phenoxide ion will only form an ester with an acyl chloride
C6H5O- + CH3COCl —> CH3COOC6H5 + Cl-
Can we make an ester by reacting phenol with a carboxylic acid?
No, this is impossible. Phenol is not a strong enough nucleophile to attack the polar atom of the ethanoyl group.
Give the observations of the following reacting with ethanol and phenol.
NaOH
NaOH in I2 (iodoform reaction)
Br2
Ethanoic acid with concentrated H2SO4
FeCl3
Concentrated H2SO4
Ethanol: Phenol:
No reaction. Phenoxide ion
Yellow ppt. No reaction
No reaction (orange). White ppt
Ester formed. No ester formed
No change (orange). Violet colour
Ethene. No reaction
Do aldehydes and ketones form hydrogen bonds?
Not in the pure state, as the H is not directly bonded to the O
Do aldehydes and ketones have an equal charge all the way through?
No, they have a permanent dipole as the electronegative O withdraws electrons from the C
Where is the boiling point of aldehydes and ketones compared to other molecules?
It’s higher than similarly large non-polar molecules but lower than for compounds that can hydrogen bond with themselves.
What is the solubility of aldehydes and ketones like?
They are soluble in water as they can form hydrogen bonds with it. Solubility increases with the length of the non-polar carbon chain.
How do we form carbonyl compounds from alcohol?
Aldehydes: oxidise primary alcohols. Ketones: secondary
Use acidified potassium dichromate or KMnO4
Conditions are important - you don’t want to go making carboxylic acids!
What does Tollen’s Reagent do?
Contains a diaminesilver ion, Ag(NH3)2(+)
This will oxidise an aldehyde in warm water to form a sliver mirror
Ketones cannot oxidise with the reagent.
What does Fehling’s reagent do?
Copper (II) ions act as a weak oxidising agent.
It will oxidise aldehydes to form a red-brown precipitate of copper (I) oxide, Cu2O
The red precipitate is not formed for ketones.
What does Brady’s reagent do?
Also known as 2,4 DNP, it tests for the presence of a carbonyl group (C=O)
A positive test form an orange precipitate. If recrystallised, the melting point can show the exact carbonyl.
The reaction consists of addition followed by substitution (condensation)
What does the iodoform test do?
Tests for the groups:
CH3 - carbonyl group - R
CH3 - C (OH going up, H going down) - R’
Positive test will give a yellow precipitate of triiodomethane CHI3
To carry out test, warm the sample with iodine in sodium hydroxide solution
Draw the displayed mechanism for addition with carbonyl compounds and hydrogen cyanide.
What kind of compound is the product? What happens if you add sulfuric or hydrochloric acid to the product and warm it?
Not typing that out. Diagram L
The product is a nitrile. If you add sulfuric or hydrochloric acid and warm, it will undergo hydrolysis and you’ll get a carboxylic acid.
Give a detailed explanation of the mechanism of the reaction between hydrogen cyanide and a carbonyl compound. Give the displayed mechanism as well.
See diagram L for the mechanism
HCN is a weak acid and can partially dissociate.
1. Add sodium cyanide to increase the number of cyanide atoms
2. The nucleophilic CN- attacks the delta+ carbon on the polar carbonyl group
3. This increases the polarisation of the carbonyl and gives O a negative charge
4. The positive H from the HCN bonds to the oxygen forming a hydroxy group
This is nucleophilic as the length of the carbon chain has increased
Is the charge constant throughout a carboxylic acid?
Not at all. The oxygen is very electronegative, so polarises the molecule strongly
Are carboxylic acids soluble in water?
Yes, but the solubility decreases as the non-polar R-group gets bigger
Can carboxylic acids hydrogen bond?
Yes, even with each other to form dimers which have higher boiling points than alcohols!
Give three ways to make carboxylic acids
- Reflux primary alcohols or aldehydes with acidified potassium dichromate
- Hydrolysis of carboxylic acid derivatives
- Oxidising compounds with methyl side chains by refluxing with alkaline potassium permanganate before acidifying
How does the acidity of carboxylic acids compare to that of alcohols and phenols?
Weak acid - only slightly ionised in aqueous solution
The proton can be donated more easily than by alcohols as the multiple oxygen atoms increase the polarity.
Same for phenols as the OH is directly on the benzene ring.
The negative charge on COO- is stabilised over three atoms
What is produced when carboxylic acids react with bases, metals, and carbonates?
Acid + base —> salt and water
Acid + metal —> salt and hydrogen
Acid + carbonate —> salt, CO2 and H2O
How does one test for a COOH group? Give the equation
Add sodium hydrogen carbonate. There will be effervescence that turns limewater cloudy.
RCOOH + NaHCO3 —> RCOONa + CO2 + H2O
How does one reduce a carboxylic acid?
NaBH4 is too weak, so we have to use LiAlH4 instead. This reacts violently with water so we do it in a fume cupboard in dry ether.
The reaction doesn’t stop at the aldehyde, as you might have guessed.
What does decarboxylation mean?
Removing CO2
How does one decarboxylise a carboxylic acid and what is this used for?
To remove CO2, heat with soda lime and a strong alkali (NaOH, perhaps?)
Can be used to identify which acid it is.
How do we make esters if not using acid chlorides?
React an alcohol with a carboxylic acid (or some derivatives)
We add H2SO4 as well to optimise the equilibrium as it:
1. Acts as a catalyst
2. Is a dehydrating agent, shifting the equilibrium to increase ester yield
How does one get a carboxylic acid from an ester? There are two ways, which is the more efficient?
You’d have to hydrolyse it by refluxing with dilute acid.
For a higher yield, reflux with a dilute alkali which reacts with the acid, shifting the equilibrium in favour of the products. Regenerate the acid later with a strong acid such as HCl. Magic!
Describe the experimental method for making esters with carboxylic acid, not acyl chloride
- Reflux the reaction in a water bath as the alcohol is flammable
- Neutralise the leftover acid using NaHCO3(aq)
- Separate the layers using a separating funnel
- Distil and dry the ester using anhydrous CaCl2
- Collect the ester at its boiling point
How do we make acid chlorides? Give the reagents, conditions, and observations. Write out a word equation.
React carboxylic acid with PCl5 (or PCl3, SOCl2)
In the fume cupboard in dry conditions
White misty fumes of hydrogen chloride are released
Ethanoic acid —> ethanoyl chloride + HCl + POCl3
How do we reform COOH from acid chlorides?
Acid chlorides react violently with water (hydrolysis) to reform carboxylic acid
What happens when we react acid chlorides and alcohols? Why would we use acid chlorides instead of carboxylic acid?
Makes an ester.
Acid chlorides are more reactive than carboxylic acids so will form esters with compounds that carboxylic acids can’t, for instance phenol.
Draw the skeletal mechanism for the reaction between acyl chloride and phenol
I’d type it out if it weren’t for those benzene rings. See diagram M
How do we form amides? Give the skeletal mechanism
We react ammonia or amines with acid chlorides
See diagram N for the mechanism
What happens when we react acid chlorides with ammonia or amines? Give the skeletal mechanism
We form an amide. For the mechanism see diagram N
Give the two-step mechanism (written) for how to make amides from carboxylic acid and ammonia.
What happens upon further dehydration?
- Carboxylic acids react with ammonia to form ammonium salts
RCOOH + NH3 —> RCOO-NH4+ - By heating the salt (dehydration) an amide is formed
RCOO-NH4+ —> RCONH2 + H2O
Further dehydration (heat with P4O10) produces a nitrile
RCOHN2 —> RC☰N
How to we get a carboxylic acid from an amide or a nitrile?
We hydrolyse it by refluxing with dilute sulphuric acid
Draw the entire journey from a carboxylic acid to a nitrile, including both directions
Diagram O :)
How do we form nitriles from halogenoalkanes?
By reacting alcoholic potassium cyanide (KCN in ethanol) with a halogenoalkane. The reaction increases the carbon chain length by one.
What happens when halogenoalkanes react with KCN in ethanol? Write out the reaction.
We form a nitrile!
R-Br + Alcoholic KCN —> R-C☰N
How do we reduce nitriles and what happens during reduction?
Nitriles can be reduced by LiAH4 (most things can, to be honest) at 0 degrees in dry ether to form an amine. Both the C and the N gain two hydrogens form the reducing agents
Draw the five organic derivatives of ammonia. What do we call them?
The first is just ammonia. We make amines. Then we just replace the hydrogens one by one with methyl groups (don’t forget the lone pair on the N) until we reach the quaternary amine which has four methyl groups and no lone pair - instead a formal positive charge on the N.
What is the shape of an amine? What happens if we add alkyl groups?
Trigonal pyramid based on a tetrahedron.
Distortion from strong repulsive forces between lone and bonded pairs
Adding alkyl groups increases the bond angle - increased inductive effects towards nitrogen make bond angles shorter and repulsion greater.
Describe the pH of amines
They are bases - their solutions in water are alkaline
Can amines hydrogen bond?
Primary and secondary can bond to each other —> high boiling points
Lower Mr amines can hydrogen bond to water
What kind of bonding is common in amines?
Bonding with electron-deficient species that possess empty orbitals (lone pair on N)
What two things does the basic strength of amines depend on?
- The availability of the lone pair
- The consequent stabilisation of the protonated amine cation by solvation of water molecules, whatever that might mean.
Something about the protonated amine stabilises by forming H-bonds with three other water molecules?
How does structure of amines change their basic strength?
In primary amines, basic strength increases with size
As the alkyl group becomes more bulky, steric hindrance decreases strength
How does the basicity of phenylamine compare to that of an aliphatic amine?
It is a weaker base because the lone pair on N interacts with the delocalised pi molecular orbitals of the benzene nucleus, rendering them less able to accept a proton
How can we make an aliphatic amine from a halogenoalkane in two steps? Equations as well please
- Convert the halogenoalkane into a nitrile by adding alcoholic KCN and warmth
R-Br —> R-C☰N + Br- - Use LiAlH4 in dry ether at 0 degrees to reduce
R-C☰N —> R-CH2-NH2
Give two ways to make an aliphatic amine from a halogenoalkane
- By converting a halogenoalkane into a nitrile and reducing that into an amine
- By reacting a halogenoalkane with ammonia in a sealed tube bomb
Give the reaction of a halogenoalkane and ammonia in a sealed tube bomb.
What happens if further reactions occur?
R-Br —(NH3 in ethanol, heat + tube)—> R-NH2 + HBR
Further reactions make secondary and tertiary amines
In short, how do we make phenylamine from nitrobenzene? Give the full equation
Now give the two-step method with two further equations
Tin and concentrated HCl (reducing agents), heat and reflux followed by NaOH
C6H5NO2 —[6H]—> C6H5NH2 + H2O
- The nitrobenzene is reduced to form a salt of the amine
C6H5NO2 —> C6H5NH3+Cl- - Add NaOH to liberate the phenylamine from its salt
C6H5NH3+Cl- + NaOH —> C6H5NH2 + NaCl + H2O
Give the detailed, 6 part method for making phenylamine from nitrobenzene
- Separate phenylamine from its impurities by steam distribution
- Reduce the solubility of phenylamine in water by adding NaCl (this is called salting out)
- Extract the mixture using ether and a separating funnel
- Dry the phenyl by adding KOH
- Remove the remaining ether by distilling it under reduced pressure
- Prevent decomposition by purifying under reduced pressure
What does an amine form a salt with? How do we liberate the amine from its salt?
Forms salts with mineral acids such as HCl and H2SO4
We can liberate the amine from its salt by adding a strong base such as NaOH
How do we test for an ammonium ion?
Add aqueous NaOH and warmth
The salt will decompose to give ammonia gas which turns red litmus paper blue
What does alkylating amines give?
Amides!
Why would we alkylate an amine to give an amide?
They can be used to identify amines due to their sharp melting point
Draw (skeletally) the reaction between phenylamine and an acyl chloride
Not typing that. See diagram P
What do we react to make an amide?
An amine and acyl chloride
What is nitrous acid. Describe its stability and how it is made. Give a written equation
Nitrous acid (HNO2) is an unstable compound made by reacting sodium nitrate (NaNO2) with hydrochloric acid in situ
NaNO2 + HCl —> HNO2 + NaCl
Aliphatic amines with nitrous acid: describe loosely (with equation) the three steps to get an alcohol
- Add HNO2 at room temperature to form an unstable diazonium ion
- This will decompose readily to release nitrogen gas (effervescence and carbonium ion formed)
- This will react with other substances to form a new compound
R-NH2 —(HNO2 room temp)—> RN(+)☰NCl(-) —> R+ + N2(g) —(OH-)—> R-OH
What will the reaction between primary aromatic amines and nitrous acid form?
A stable diazonium ion but only when the nitrous acid is at 0-5 degrees.
Which alkaline solution give which colours when reacted with diazonium salts?
Phenol with NaOH is yellow, 2-naphthol with NaOH is red
Give the process to get from diazonium salts to an azo dye skeletally. Give a short description
The diazonium ion is an electrophile and will attach the reactive nucleophilic site (4 for phenol and 2 for 2-naphthol)
The mechanism is in diagram Q
What is an azo-coupling and what is so special about it.
An azo-coupling is a N=N and it absorbs certain wavelengths of light, giving azo-dyes their colour
What is a chromophore?
A structural unit responsible for the absorption of certain wavelengths of electromagnetic radiation
What are the names of the following amino acids?
NH2 — CH2 — COOH
NH2 — CH(CH3) — COOH
Glycine and alanine
What makes an amino acid more soluble?
If the acidity or basicity of the solution increases
Are amino acids acids or bases?
They are amphoteric because of the two ionisable groups in the same molecule
Draw the normal and zwitterion forms of an amino acid
Diagram R :)
Draw an amino acid in acidic conditions. What is the overall charge? Which part is the base?
NH3+ — CH2 — COOH
Overall charge is positive, the amino acid is a cation
The COO- has picked up a proton to become COOH, thus it is the base
Draw an amino acid in basic conditions. What is the overall charge? Which part is the acid?
NH2 — CH2 — COO-
Overall negative charge, the amino acid is an anion
The OH- removed the H+ from NH3+ so that is the acid
How are peptides formed? Draw the displayed reaction
When amino acids polymerise by condensation. For the reaction see diagram S
Draw the displayed formation of nylon 6,6
Diagram T :)
Give four similarities between polymers (nylon) and polypeptides (proteins)
- Formed by condensation polymerisation
- Both have peptide-linked monomers
- Can be turned into fibres (nylon, wool, silk…)
- Macromolecules of large molecular mass
Give three differences between nylon-like polymers and protein-like polypeptides
- Nylon has two monomers with repeating unit AB while a polypeptide has many different monomers (amino acids)
- Protein peptide linkage is closely repeated (about 1 carbon) whereas nylon is repeated after 6
- The main forces between nylon chains are Van der Walls which makes them strong and tough. Proteins have many different forces such as hydrogen bonding and dipole-dipole
Describe the primary structure of proteins
A chain of amino acid residues
Describe the secondary structures of proteins. Give properties and examples of each.
Alpha helix: if the side groups are large, steric hindrance allowed H-bonding only on every fourth amino acid. Strong and elastic. Keratin (hair), myosin (muscle), and fibrogen (blood)
Beta pleated: with small side groups and minimal steric hinderance. Strong but inelastic. Silk, fibroin
Describe the tertiary structure of proteins and give two examples
Bending and folding over of helices to create a permanent, stable shape. Shape depends on type fo side groups of amino acid residues and their many attractive forces.
E.g., enzymes and myoglobin
Give two roles of proteins in living systems
- Insulin (hormone) controls glucose levels in blood
- Haemoglobin transports blood around the body
What happens when proteins are denatured and how does this occur
Destroys secondary and tertiary structures but leaves primary structure intact
Caused by heating above 70 degrees, adding strong acids or bases, or dissolving in organic solvents
