A2 Organic Chemistry Flashcards
Why is Benzene considered neither saturated nor unsaturated?
Benzene is considered neither saturated nor unsaturated because when they are reacted with bromine, and cold KMnO4, no reaction occurs which shows that they are saturated but when benzene is reacted with H2 in the presence of a nickel catalyst and T=150 and P=100atm, reaction occurs which shows that they are saturated.
So in conclusion benzene is neither saturated nor unsaturated.
List the proofs that show that Benzene is neither saturated nor unsaturated.
Three proofs that show that Benzene is neither saturated nor unsaturated:
- the C-C bond lengths in a delocalized system are equal i.e. 1.40A which is neither of saturated C-C single bond (1.30A) nor of unsaturated C=C double bond (1.50A).
- benzene does not react with Br+ under normal conditions.
- the hydrogenation energies of benzene are lower than expected, which shows that benzene is more stable than cyclohexatriene.
List the bonding characteristics of Benzene.
- all Carbon in benzene are sp2 hybridized and hence there is one unhybrid pz orbital.
- there are 12 sigma bonds, 6 C-C sigma bonds while 6 C-H sigma bonds.
- there are three pi bonds which continuously delocalize over the benzene ring.
- all C-C bond lengths are same because of delocalization of pi electrons.
- benzene is planar with a bond angle of 120 degree.
Compare Benzene with Alkenes.
Benzene does not undergo reactions like Alkene due to greater stablity.
Benzene is more stable than Alkenes due to delocalisation of pi electrons.
Reactions of Benzenes are less exothermic than Alkenes which shows its greater stablity.
Product of Exothermic reactions of Benzene are stable due to lower energy, hence product of more exothermic reactions will be most stable.
What types of reactions does Arenes/Aromatic Compounds undergo and why?
Arenes/Aromatic Compounds undergo electrophilic reactions because of the presence of delocalised electrons over the Benzene ring.
Arenes/Aromatic undergoes electrophilic substitution reaction rather than electrophilic addition to maintain it’s stablity.
Draw the mechanism of electrophilic substitution reaction in benzene.
Attach pic from web
Recall that Elecrophilic substitution reaction occurs in two steps, why is the first step slow and the second fast?
In the first step the aromatic stable system is getting disturbed, due to the attacks of the electrophile so this step is slow.
While in the second step the system is returning to stable position so it is fast.
Recall the energy vs. reaction graph of benzene.
Attach gif from web
What are the examples of addition and substitution reactions of benzene, and what reagents and conditions are needed for these reactions to occur.
Addition reactions: Halogenation: U.V light. Hydrogenation: Ni + 150<sup>0</sup>C + 100atm. Electrophilic substitution reactions: Chlorination: AlCl3 in dark. Bromination: AlCl3/FeBr3 in dark. Nitration: HNO3 + H2SO4, T=55~60 degree C. Alkylation: AlCl3. Acylation: AlCl3.
How is Benzene prepared?
- benzene can be prepare from crude oils.
- when Aliphatic hydrocarbons undergo aromatization, arenes are formed.
Catalyst used for Aromatization/Dehydrogenation is Pt.Al2O3, T=500 degree C and P=15atm.
What are the properties of Benzene?
- benzene is a colourless poisonous liquid.
- benzene has lower b.p than other Aromatic compounds due to weak Vander waal forces.
- benzene is non-polar hence miscible with organic solvents and immescible with polar solvents.
What are the two types of orientation directors? And what do they do?
There are two types of orientatiion directors:
2,4,6 directors (are electron donating and activating groups because they create negative charge on benzene ring and hence reactivity of the ring is increased towards the electrophile).
3,5 diectors (are electron withdrawing and deactivating groups because they create positive charge on the benzene ring and decrease the reactivity of the benzene ring towards the electrophile).
What is the Inductive effect?
The polarisation of sigma bonds due to the presence of an electronegative atom is called inductive effect.
All electron withdrawing groups create neative inductive effect while electron donating groups create positive inductive effect.
Recall the reactions of methyl benzene and what are the two categories the reactions are categorized in?
The reactions of Methyl benzene are categorised into the following two categories:
Reactions with ring:
Halogenation: AlCl3 in dark.
Nitration: conc.HNO3(reagent) + conc.H2SO4(catalyst), T=30-55oC, if T>55oC then more number of NO2+substitution.
Alkylation: AlCl3.
Reactions with side chain:
Oxidation of side chain: H+/KMnO4.
acidified K2Cr2O7 cannot oxidize alkyl side chain.
Tertiary alklyl chain cannot be oxidized.
Halogenation: U.V light.
Hydrolysis: (aq)NaOH with reflux.
Why Halogens are an exception to the orientation directors?
All 2,4,6 directors are electron donating and activating groups except halogens whlch are 2,4,6 directors but electron withdrawing and deactivating groups due to higher electronegativity, they create negative inductive effect.
What are phenols and list their properties?
Phenols are compounds which have hydroxyl group directly attached to the Aromatic ring are called phenols.
Phenols are crystalline solids.
They have slightly higher mp, bp than other arenes because of hydrogen bonding.
Phenols are slightly soluble in water.
Phenols are weak acids and hence incompletely dissociate in water.
Recall the concept of strength of acids.
Every strong acid has a weak and stable conjugate base.
Every weak acid has a strong and unstable conjugate base.
Strength of acid depends upon the stablity of the conjugate base.
Compare the acidity of phenol, ethanol and carboxylic acids.
Every phenol is stronger acid than ethanol because the phenoxide ion(conjugate base) lone pair of oxygen atom is continuously delocalised over the benzene ring., hence stablity of the conjugate base increases.
In ethanoxide ion the electron donating alkyl group increases the electron density of the oxgen atom and hence increases the availablity of oxygen atom to accept proton., consequently stablity of the conjugate base decreases.
Carboxylic acid is stronger acid than phenol and ethanol because the ethanoate ion is much more stable than phenoxide and ethanoxide ion. This is because the lone pair of oxygen atom is continuously delocalised over the carbonyl group.
What are the electron withdrawing and electron donating groups and what is their effect on the acidity of phenol?
Electron withdrawing groups increase the acidity of phenol.
This is because the electron withdrawing groups decrease the availablity of lone pair of oxygen in conjugate base hence makes it less available to accept proton, consequently stablity of the conjugate base increases.
OR
Electron withdrawing group polarises the OH bond, hence the OH bond becomes weaker consequently phenol releases it’s proton readily.
Electron donating groups decrease the acidity of phenols.
This is because electron donating groups increase the availability of lone pair of oxygen atom in conjugate base hence makes it less available to accept proton, consequently stability of the conjugate base decreases.
Recall the reactions of phenol and what the two types of tests for phenol are?
*Bromination: CCl4 inert solvent Br2(l) and (aq).
Nitration: conc.HNO3 and dilute HNO3
Esterification: reaction of phenol with acyl chloride.
*Alternate test for phenol:FeCl3 added to phenol, purple colouration. PH=7
What are amines and are they acidic basic or neutral?
Amines are derivatives of ammonia in which one or more hydrogen atoms are replaced by Alkyl or Aryl group. They are basic like ammonia because of the presence of lone pair of nitrogen atom, they may accept proton.
Compare the basicity of Ammonia, primary, secondary, tertiary amines and Aryl amines.
Amines are more basic than ammonia because of the electron donating Alkyl group which increases the availability of the lone pair of nitrogen hence makes it more available to accept proton.
Secondary Amine is more basic than primary Amine because two electron donating Alkyl group which increases the availablity of lone pair of nitrogen the most.
Tertiay Amine is less basic than primary and secondary because of three bulky Alkyl groups which create hinderence for nitrogen to accept proton.
Aryl Amine are least basic because lone pair of nnitrogen delocised over benzene ring.
What are the four ways Amines can be prepared (also mention the reagents and conditions needed to prepare them)?
Amines can be prepared by the reduction of nitriles and amides using (LiAlH4 in ether).
Adding (conc.NH3 in ethanol) to Alkyl halides.
Reduction of nitrobenzene using Sn(tin) in conc.HCl with reflux.
Recall the reactions of Amines (also mention the reagents and Conditions needed)?
Neutralisation
Secondary/Tertiary amine formation
Alkyl halide is used.
Amide formation
Carboxylic acid or Acyl halide is used.
*Bromination: test for Aniline; white ppt.
Diazonium compound formation: Aniline + HCl + NaNO2, T=0-5°C.
