Hydrocarbons Flashcards
Preparing alkanes from carboxylic acids
Sodium salts with soda lime give one less carbon atom (decarboxylation)
Decarboxylation
Removal of a mole of CO2 from an organic compound
Why can’t Wurtz reaction have an odd number of carbon atoms
It will lead to the formation of mixture of two substances
Why does Kolbe’s use even number of carbon atoms
The mechanism involves the combination of two free radicals
Preparing alkanes from alkyl halides
- On reduction with Zn and dil. HCl give alkanes
- On treatment with sodium metal gives Wurtz reaction (dry ether)
BP of alkanes dependent on
- Molecular mass
- Branching - Increase = Low SA = Low BP
- Side Chain - Increase = High SA = High BP
Explain Kolbe’s electrolytic method
- Aq solution of sodium on electrolysis gives alkane with even no.
- Anode: Free radical + CO2
- Cathode: H2 and OH
Explain substitution reactions (alkanes)
Halogenation takes place at higher temperatures or in the presence of diffused sunlight
Explain initiation propagation and termination of substition with alkanes
- Initiated by homolysis of chlorine molecule - Cl Cl bond is easier to break
- Propagated by breaking CH bond to generate methyl free radical with more liberation
- Reaction stops after some time
Possible chain terminating steps
Cl + Cl –> Cl - Cl
H3C + CH3 –> H3C - CH3
H3Cl + Cl –> H3C + Cl
Explain combustion of alkanes + general formula
Completely oxidized to carbon dioxide and water with the evolution of large amount of heat
CnH2n+2 + (3n+1)/2 O2 –> nCO2 + (n+1)H2O
General combustion formula:
1. Alkene
2. Alkyne
- O has (3n / 2)
- O has (3n-1)/2
Combustion formula using x and y
CxHy + (x + y/4)O2 –> xCO2 + y/2 H2O
Isomerization of alkanes
In presence of anhydrous AlCl3 and HCl isomerize to branched chain alkanes
Aromatization of alkanes
- Having 6 or more C atoms in presence of oxides of vanadium get dehydorgenated and cyclised
- 773K and 10-20 atm
Alkanes with steam
With steam in the presence of nickel catalyst to form carbon monoxide and dihydrogen
Cis-Isomer
Type of geometrical isomer which same atoms; bonds but diff arrangement
X - C - Y
|| (On the same side)
X - C - Y
Why is trans isomer less polar
As trans isomer has the group on different sides, the dipoles cancel out
Why does trans isomer have higher MP?
Trans isomer is more symmetrical
Why do alkanes not have free rotation?
Electron distriubtion of sigma molecular orbital is symmmetrical around internuclear axis
Conformations
Spatial arrangmements of atoms which can be converted into one another by rotation around a C-C bond
Newman projections
- Molecule viewed head on
- Carbon atom depicted as a point (nearer)
- Drawn at 120 degree angles
Torsional strain
Small energy barrier due to weak repulsive interaction between adjacent bond
Explain eclipsed, staggered and skewed conformation
- Eclipsed: Carbons as close as together
- Staggered: As far apart as possible
- Skewed: Intermediate conformation
Sawhorse projections
- Molecule viewed along molecular axis
- Projected on paper by drawing central C-C bond somewhat longer straight line
- Lines are inclined at an angle of 120 to each other
- Has both eclipsed and staggered conformations
Preparation of alkenes from alkynes
+ H2 Catalyst: Pd / C (cis isomer)
Catalyst: Na (trans isomer)
Reaction learn
Preparation of alkene from alkyl halide
Heating with alcoholic KOH (dehyrohalogenation & beta - elimination)
Reaction rate for alkyl and halogens
Alkyl : Tertiary > Sec > Pri
Halogens: Iodine > Br > Cl
Prep of alkene from vicinal dihalide
Add Zn forms ZnX2 (dehalogenation)
Vicinal groups
Groups attached to two separate carbon atoms that are adjacent
Geminal groups
Groups which are attached to the same carbon atom
Prep of alkene from alcohols
Acidic dehydration (add conc. H2SO4) gives water
Beta elimination
Why does cis isomer have higher BP
Cis has higher BP due to more polar nature leading to stronger intermolecular dipole-dipole interaction, this requiring more heat energy to separate them
Why is benzene so stable?
- Delocalization of pi electrons
- Presence of resonance
Increase in Size increases
BP
Addition of halogens to alkenes
Form vicinal dihalides (bromine is discharged) - unsaturation
Addition of hydrogen halides
Form alkyl halides
Order of reactivity is HI>HBr > HCl
Explain Markovnikov mechanism
- Either form 2-bromopropane or 1-bromopropane
- 2-bromopropane has more stable secondary carbocation
What is markovnikov rule
States that the negative part of the addendum gets attached to that carbon atom which hass lesser hydrogen atoms
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Preparing alkanes from unsaturated hydrocarbons
Pt / Pd / Ni used as catalysts to convert / hydrogenation
Explain kharash / anti-markivnikov effect
- With the presence of peroxide
- Secondary free radical is more stable
Addition of sulphuric acid to alkenes
Addition form alkyl hydrogen sulphate (breaking double bond with OSOOOH)
Addition of water to alkene
In presence of conc. H2SO4 form alcohols
Oxidation of alkenes
React with cold, dilute KMNO4 form vicinal gylocols
Used for unsaturation
Arrange the halogens F2, Cl2, Br2 and I2 in order of their increasing reactivity with alkanes
F2 > Cl2 > Br2 > I2 (decrease in electronegativity down the group)
Explain ozonolysis of alkenes
- Forms ozonide and then cleavage of the ozonide by Zn- H2O to smaller molecules
Polymerization of alkenes
Combination of large number of ethene molecules at high pressure, high temperature and in presence of a catalyst
Forming alkyness from vicinal dihalides
Undergo treatment with alcoholic potassium hydroxide to undergo dehydrohalogenation
Preparation of alkyne from calcium carbide
- Heating quick lime with coke
- Then heating to form calcium carbide reacting with water
Why is ethyne more acidic than ethene
- Ethyne has maximum sp character which have higher electronegativity hence attract shared pair of CH bond
Addition of halogens to alkynes
- Reddish orange colour of the solution of bromine in carbon tetrachloride is decolourised
Addition of hydrogen halides (to alkynes)
Two molecules of hydrogen halides add to alkynes to form geminal dihalides
Addition of water to alkynes
- Water with dilute sulphuric acid at 333K forms carbonyl compounds
- It forms alcohol
- Alcohol loses the H group to form aldehyde or ketone
Cyclic polymerization of ethyne
Undergoes cyclic polymerization ot 873K to form benzene
Conditions for aromaticity
- Planarity
- Complete delocalization of pi electrons
- Presence of (4n+2) pi electronsc
Decarboxylation of aromatic acids
Sodium salt of benzoic acid on heating with soda lime gives benzene
Reduction of phenol
Reduced to benzene by passing its vapours over heated zinc dust
Nitration of benzene
Benzene heated with Conc. HNO3 and conc. H2SO4
Forms nitrobenzene
Halogenation of benzene
In presence of a lewis acid (FeCl3, FeBr3 AlCl3)
Sulphonation
Replacement of a hydrogen atom by sulphonic acid
Heating benzne with sulphuric acid
Friedel-Crafts alkylation
Benzene with alkyl halide in presence of anhydrous AlCl3
Friedel crafts acylation
Reaction of benzene with acyl halide in presence of AlCl3
Why do alkenes undergo electrophilic addition while arenes undergo substitution?
- Alkenes and arenes each have electrophilic reaction (electron rich)
- Alkenes are unsaturated, hence undergo addition
- Arenes would break their resonance energy if they undergo addition and hence go substitution where it maintains its energy
Why is staggered conformation more free?
- Staggered has minimum repulsive forces
- Minimum energy and maximum stability
- Magnitude of torsional strain is much lesser
Ortho and para directing groups
- Electron density is more on o & p positiions
- -I effect is slightly reduced
- However, overall electron density increases due to resonance
- Electrophiles attack
What are electrophiles
- Accepts electrons (positively or neutral)
- Electron deficient; lewis acids
- Eg: -OH, -NH2, -CH3, -OCH3
Nucleophiles
- Donates electrons (negative or neutral)
- Electron rich; lewis bases
- Eg: -NO2, -CN, -CHO, -COR
Meta directing groups
- Nitro group reduces electron due to strong -I effect
- Overall density decreases making further substitution difficult
+I vs -I effect
- Electron density is more towards the electronegative atom
- +I involves pulling the electron density causing polarization - like Cl, F, Br
How to determine reactivity towards an electrophile (Benzene and substit)
-I -R = low reactivity (OCH3)
-I +R / no R = mid (Cl)
+I +R = high reactivity (NO2)
Why are haloarenes ortho para directing
-I > +R
* -I causes deactivating effect (causes low electron density)
* However, +R combats it, increasing electron density at O- and P- positions more than meta
How to convert benzene into p-nitrobenzene
- Benzene + Br2
- Add conc. HNO3 for nitrogen
- Bromine is an ortho-para directing group where as NO2 is a meta-directing group
Why does benzene undergo electrophilic substitutions easier than additions?
Due to presence of 6pi electrons, behaves as a rich source of electrons thus being easily attacked by reagents deficient in electrons
