A2 - Unit 1 - Rings, acids and amines Flashcards
What are arenes?
Aromatic hydrocarbons containing one or more benzene rings
What is an aromatic compound?
Any compound with a benzene ring
What do you call a benzene ring with an NO2 group attached?
Nitrobenzene
What do you call a benzene ring with a Br attached?
Bromobenzene
What do you call a benzene ring with a Cl attached?
Chlorobenzene
What do you call a benzene ring with a C2H5 group attached?
Ethylbenzene
Give some properties of benzene on its own
Colourless liquid
Sweet odour
Highly flammable
Why did Kekule’s model for benzene fail to explain benzene’s low chemical reactivity?
If C=C double bonds were present then benzene would react in a similar way to alkenes
Each C=C would be expected to react with bromine water, decolourising it
Why did Kekule’s model for benzene not match up with X ray images of benzene?
C=C bonds and C-C bonds have different lengths and the X ray image revealed that the molecule was symmetrical and all the bond lengths were exactly the same
Why did Kekule’s model for benzene not match up with the enthalpy change of hydrogenation?
The enthalpy change of hydrogenation for cyclohexane with one double bond is -120 kJ mol-1 so for benzene with three double bonds should be -360 kJ mol-1
However the actual enthalpy of hydrogenation is -208 kJ mol-1 which does not match with what was expected
Which three factors disproved the Kekule model for benzene?
Bond lengths
Low reactivity
Hydrogenation of Benzene
What are delocalised electrons?
Electrons which are shared between more than two atoms
What features does the delocalised model of benzene have?
C6H6
planar hexagonal ring
trigonal planar 120degrees around each carbon atom
four outer shell electrons
three electrons bond to two carbon atoms and a hydrogen
three bonds in this plane are called sigma bonds
leaves the fourth electron in a 2p orbital above and below the plane of the carbon atoms
electron in the p orbital of carbon atom, overlaps with the electrons in the p orbitals of neighbouring carbon atoms, results in ring of electron density above and below the plane of the carbon atoms
^ produces pi bonds, not just held between carbon atoms, spread over whole ring, said to be delocalised
Under normal conditions what does benzene not do?
Decolourise bromine
React with strong acid such as HCl
Reacts with halogens chlorine, bromine and iodine
What is an addition reaction?
One in which a reactant is added to an unsaturated molecule to make a saturated molecule
What is a substitution reaction?
One in which an atom or group of atoms is replaced with a different atom or group of atoms
Why does benzene not perform any addition reactions?
Electrons from the delocalised system would need to bond to the atom or group of atoms resulting in the product being less stable than benzene, and the reaction would not be energetically favourable
What reaction does benzene take part in instead of addition reactions?
Substitution reactions
one of benzenes hydrogen atoms is replaced by another atom or group of atoms
the organic product formed retains the delocalisation and hence the stability of the benzene ring
What is an electrophile?
An atom or group of atoms that is attracted to an electron rich centre, where it accepts a pair of electrons to form a new covalent bond
What specific reaction does benzene part take in?
Electrophilic substitution
What are the reagents and conditions for the nitration of benzene?
Concentrated HNO3
Concentrated H2SO4
50 degrees C
Give the equation for the preparation of nitrobenzene
C6H6 + HNO3 —-> C6H5NO2 + H2O
What practical is carried out when nitrating benzene?
Pear shaped flask
Conc nitric acid and conc sulphuric acid added while cooling the mixture in a beaker of cold water
benzene is added carefully to the nitrating mixture keeping the temp below 50 degrees
Reflux
When nitrating benzene for a mono-substitution reaction why do you need to ensure the temperature stays below 50 degrees?
Because if the mixture gets any hotter, then more than one nitro group may be substituted onto the benzene ring
What can be used to make a halogen react with benzene?
A halogen carrier
Give an example of a possible halogen carrier
AlCl3 or FeCl3 (swapping Cl for the halogen you happen to be using)
How do you carry out the halogenation of benzene?
Form the electrophile: Br2 + FeBr3 --> Br+ + FeBr4- The electrophile reacts with benzene Reform the catalyst: H+ from benzene and the FeBr4- react together making FeBr3 again and HBr
What is electrophilic substitution?
A type of substitution reaction in which an electrophile is attracted to an electron-rich centre or atom, where it accepts a pair of electrons to form a new covalent bond
What is a reaction mechanism?
A series of steps that, together make up the overall reaction
What is a curly arrow?
A symbol used in reaction mechanisms to show the movement of an electron pair in the breaking or formation of covalent bonds
What happens when bromine approaches an alkene such as cyclohexene?
The double bond will interact with the bromine and attach to make 1,2-dibromocyclohexane
1. The pi bond repels the electrons in the Br-Br bond
2. Induced dipole
3. One end of molecule slightly negative
Pi electron pair attracted to slightly positive bromine
4. Positively charged carbocation formed
5. Br-Br bond breaks forming Br- ion
6. Br- ions attracted towards intermediate carbocation forming covalent bond
Give some properties of phenol
Solid at room temp
Slightly soluble in water
However presence of benzene makes phenols less soluble in water than alcohol
When dissolved in water, what does phenol form??
Weak acid
What happens when a reactive metal such as sodium is added to phenol?
The metal effervesces producing hydrogen gas
Organic salt product made
What reaction happens when you add phenol and bromine together?
Electrophilic substitution
When you add bromine to phenol where does the bromine add with regards to the OH group?
2,4,6
Why does bromine react more readily with phenol than benzene?
Lone pair of electrons on the oxygen is drawn into the phenol group and drawn into the benzene ring
Creates higher electron density and ring is activated
Increased electron density polarises bromine molecules, which are attracted more strongly towards the ring structure than in benzene
Give some uses of phenol
Antiseptic
Synthesis of dyes
Manufacture of pharmaceuticals (e.g. Aspirin)
What is the use of alkyl phenols?
Found in surfactants and detergents
What is the use of chlorophenols?
Antiseptics and disinfectants such as TCP and Dettol
What is the use for salicylic acid?
Preparation of aspirin and other pharmaceuticals
What is the use for bisphenols?
Production of epoxy resins for paints
What is a functional group??
The part of an organic molecule responsible for its chemical reactions
What is electronegativity?
A measure of the attraction of a bonded atom for the pair of electrons in a covalent bond
What functional group do all ketones and aldehydes contain?
Carbonyl
C=O
Why does a carbonyl bond not react the same as a double bond in an alkene?
Because the oxygen atom is more electronegative than the carbon atom
This creates a dipole in the C=O bond because the bonded electrons are attracted towards the oxygen
What is a redox reaction?
A reaction in which both reduction and oxidation take place
What is reflux?
The continual boiling and condensing of a reaction mixture to ensure that the reaction takes place without the contents of the flask boiling dry
How do you carry out the oxidation of a primary alcohols?
Distil with K2Cr2O7 and H2SO4 to form an aldehyde
Reflux with K2Cr2O7 and H2SO4 to form a carboxylic acid
How do you carry out the oxidation of secondary alcohols?
Oxidise with K2Cr2O7 to form a ketone
How do you oxidise an aldehyde, and how do you know when the reaction is complete?
Oxidise under reflux with K2Cr2O7 and the mixture will turn from orange to green
How do you reduce an aldehyde?
Reduced to primary alcohols
Using NaBH4
How do you reduce ketones?
Reduced to secondary alcohols
Using NaBH4
What is a nucleophile?
An electron pair donor
What is a curly arrow?
A symbol used in reaction mechanisms to show the movement of an electrons pair in the breaking or formation of a covalent bond
Explain a nucleophilic addition reaction
Aldehydes and ketones react with NaBH4 in a nucleophilic addition reaction
BH4- ion is the source of the H- ion which acts as the nucleophile
1. electron-deficient carbon atom in the polar C=O is attacked by the H- ion
2. Lone pair of electrons from the H- forms a bond with the carbon atom
3. at the same time the pi bond in the C=O bond breaks to produce a negatively charged intermediate
4. intermediate donates an electron pair to a hydrogen atom of an H2O molecule forming a dative covalent bond and a hydroxide ion OH-
5. alcohol product
What reagent is used to detect the presence of an aldehyde or a ketone?
2,4-DNP
What is Brady’s reagent?
A solution of 2,4-DNP, methanol and sulphuric acid, when this solution is added to an aldehyde or a ketone a yellow or orange precipitate is formed
How do you make Tollen’s reagent?
Aqueous sodium hydroxide added to silver nitrate until a brown precipitate of silver iodide is formed
Dilute aqueous ammonia is then added until the precipitate just dissolves
What reagent can be used to distinguish between aldehydes and ketones?
Tollen’s reagent
How does Tollen’s reagent help you to distinguish between aldehydes and ketones?
Tollen’s is a weak oxidising agent and therefore aldehydes are easily oxidised to carboxylic acid and a ‘silver mirror’ is observed
and ketones are not so no reaction is observed
Once a carbonyl has been identified as an aldehyde or a ketone, how do you then positively identify the exact carbonyl compound?
Take the yellow orange solid from the test with Brady’s, filter, recrystallise to produce a purified sample of yellow or orange crystals
Filtered and allowed to dry
Melting point measured and recorded
Melting point compared to a database to identify the original aldehyde or ketone
Which carboxylic acids are very soluble in water, and why is this?
Compounds with between one and four carbon atoms
Highly polar C=O and O-H allow the carboxylic acid molecules to form hydrogen bonds with water molecules
What happens to the solubility of carboxylic acids as the number of carbon atoms in the compounds increase?
The solubility decreases, this is as a result of the longer non-polar hydrocarbon chain in the molecule, which does not interact with water molecules
What are the salts formed with carboxylic acids known as?
Carboxylates
Carboxylic acid + metal = ?
Salt and hydrogen gas
Observation: effervescence
Carboxylic acid + base = ?
Salt and water
Carboxylic acid + carbonate = ?
Salt + carbon dioxide + water
What is esterification?
The reaction of an alcohol with a carboxylic acid to produce and ester and water
How do you make an ester?
Reacting a carboxylic acid and an alcohol in the presence of an acid catalyst
or
Gently heating an acid anhydride with an alcohol (this method gives a much better yield) creating an ester and a weak acid
What is hydrolysis?
A reaction with water or hydroxide ions that breaks a chemical compound into two compounds
What reaction is the reverse of esterification?
Ester hydrolysis
What happens in the acid hydrolysis of esters?
Ester is heated up under reflux with dilute sulphuric acid or dilute hydrochloric acid
Ester is broken down by water with the acid acting as a catalyst
Products: Weak acid + primary alcohol
What happens in the alkaline hydrolysis of esters?
Aqueous sodium hydroxide is refluxed with the ester
Reaction is non-reversible and leads to the formation of the sodium salt of the carboxylic acid, and a primary alcohol
What is the alkaline hydrolysis of esters also known as?
Saponification
Is the acid hydrolysis of esters reversible or non-reversible?
Reversible
Is the alkaline hydrolysis of esters reversible or non-reversible?
Non-reversible
What are esters generally used for?
Perfumes and flavourings
What are fatty acids made from?
They are esters of long-chain carboxylic acids
What is glycerol also called?
Propane-1,2,3-triol
What are mono-unsaturated fats?
Unsaturated fats with one double bond in the hydrocarbon chain, the double bond normally being found between carbon-9 and carbon-10
Which two isomeric forms exist in unsaturated fats?
Cis and trans
Out of both cis and trans fats, which ones increase the risk of coronary heart disease?
Trans-fats
Where are the hydrogens placed in a cis configuration of an unsaturated fat?
They are together on the same side of the double bond
Where are the hydrogens placed in the trans configuration of an unsaturated fat?
They are on opposite sides of the double bond
In nature, which form of fatty acid is most common?
The cis form
What property is a cis fatty acid at room temperature?
The unsaturated fatty acid molecules cannot pack closely together so cis fatty acids exist as liquids at room temperature
What property is a trans fatty acid at room temperature?
The trans form of a fatty acid is quite linear in its structure so the fatty acid chains can pack together closely, this leads to a higher melting point than the cis isomer meaning it is usually solid at room temperature
What is low density lipoprotein?
It is responsible for carrying cholesterol and triglycerides from the liver to the tissues
What is high density lipoprotein?
Can remove cholesterol from the arteries and transport it back to the liver for excretion or re-utilisation
Which type of lipoprotein (LDL or HDL) is known as ‘good’ lipoprotein?
HDL’s
LDL’s deposit lipids on the artery walls, building up fatty deposits and restricting blood flow
How do you create biodiesel?
A fatty acid and methanol or ethanol
Sodium hydroxide catalyst
Glycerol is also formed and sold to the pharmaceutical or cosmetics industry
This improves the atom economy of the process
What is the process of making biodiesel called?
Transesterification
What is the difference between primary, secondary and tertiary amines?
Primary - only one H from ammonia is swapped out for a carbon chain
Secondary - 2 hydrogen atoms are replaced
and Tertiary - all three hydrogen atoms from ammonia are replaced
How would you name a primary amine?
If there are four carbons on the alkyl chain, it is given the name butyl and the ending -amine is added as a suffix to give the name:
Butylamine
How would you name a secondary amine?
The longest alkyl chain has three carbons, propyl
The suffix is added -amine
The shorter alkyl chain contains one carbon connected to the N atom so N-methyl is added as a prefix
So: N-methylpropylamine
What properties of amines make them weak bases?
A lone pair of electrons on the nitrogen atom
Can accept a proton, H+
When a base accepts a proton in an amine what happens?
A dative covalent bond forms between the lone pair of the nitrogen and the proton
When you react an acid which an amine what happens?
It forms an ammonium salt
How can you make aliphatic amines?
By warming halogenoalkanes gently with an excess of ammonia, using ethanol as a solvent
How can nitrobenzene and other nitroarenes be reduced?
By using a mixture of tin and conc HCl
Heated under reflux, followed by neutralisation of the excess HCl
What product is formed when you reduce a nitroarene?
An aromatic amine and water
What are the two steps in the industrial preparation of dyestuffs?
Diazotisation
Coupling reactions
What is diazotisation?
Phenylamine + HNO2 + 2HCl (below 10 degrees) —-> diazonium salt + 2H2O
What is coupling?
Diazonium salt + phenol + NaOH
The two benzene rings are joined through the N=N functional group
