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Chem - Gr11 > Hydrocarbons > Flashcards

Hydrocarbons Flashcards

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1
Q

Preparing alkanes from carboxylic acids

A

Sodium salts with soda lime give one less carbon atom (decarboxylation)

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2
Q

Decarboxylation

A

Removal of a mole of CO2 from an organic compound

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3
Q

Why can’t Wurtz reaction have an odd number of carbon atoms

A

It will lead to the formation of mixture of two substances

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4
Q

Why does Kolbe’s use even number of carbon atoms

A

The mechanism involves the combination of two free radicals

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5
Q

Preparing alkanes from alkyl halides

A
  1. On reduction with Zn and dil. HCl give alkanes
  2. On treatment with sodium metal gives Wurtz reaction (dry ether)
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6
Q

BP of alkanes dependent on

A
  1. Molecular mass
  2. Branching - Increase = Low SA = Low BP
  3. Side Chain - Increase = High SA = High BP
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7
Q

Explain Kolbe’s electrolytic method

A
  1. Aq solution of sodium on electrolysis gives alkane with even no.
  2. Anode: Free radical + CO2
  3. Cathode: H2 and OH
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8
Q

Explain substitution reactions (alkanes)

A

Halogenation takes place at higher temperatures or in the presence of diffused sunlight

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9
Q

Explain initiation propagation and termination of substition with alkanes

A
  1. Initiated by homolysis of chlorine molecule - Cl Cl bond is easier to break
  2. Propagated by breaking CH bond to generate methyl free radical with more liberation
  3. Reaction stops after some time
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10
Q

Possible chain terminating steps

A

Cl + Cl –> Cl - Cl
H3C + CH3 –> H3C - CH3
H3Cl + Cl –> H3C + Cl

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11
Q

Explain combustion of alkanes + general formula

A

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

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12
Q

General combustion formula:
1. Alkene
2. Alkyne

A
  1. O has (3n / 2)
  2. O has (3n-1)/2
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13
Q

Combustion formula using x and y

A

CxHy + (x + y/4)O2 –> xCO2 + y/2 H2O

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14
Q

Isomerization of alkanes

A

In presence of anhydrous AlCl3 and HCl isomerize to branched chain alkanes

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15
Q

Aromatization of alkanes

A
  • Having 6 or more C atoms in presence of oxides of vanadium get dehydorgenated and cyclised
  • 773K and 10-20 atm
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16
Q

Alkanes with steam

A

With steam in the presence of nickel catalyst to form carbon monoxide and dihydrogen

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17
Q

Cis-Isomer

A

Type of geometrical isomer which same atoms; bonds but diff arrangement

X - C - Y
|| (On the same side)
X - C - Y

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18
Q

Why is trans isomer less polar

A

As trans isomer has the group on different sides, the dipoles cancel out

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19
Q

Why does trans isomer have higher MP?

A

Trans isomer is more symmetrical

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20
Q

Why do alkanes not have free rotation?

A

Electron distriubtion of sigma molecular orbital is symmmetrical around internuclear axis

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21
Q

Conformations

A

Spatial arrangmements of atoms which can be converted into one another by rotation around a C-C bond

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22
Q

Newman projections

A
  • Molecule viewed head on
  • Carbon atom depicted as a point (nearer)
  • Drawn at 120 degree angles
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23
Q

Torsional strain

A

Small energy barrier due to weak repulsive interaction between adjacent bond

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24
Q

Explain eclipsed, staggered and skewed conformation

A
  • Eclipsed: Carbons as close as together
  • Staggered: As far apart as possible
  • Skewed: Intermediate conformation
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25
Q

Sawhorse projections

A
  1. Molecule viewed along molecular axis
  2. Projected on paper by drawing central C-C bond somewhat longer straight line
  3. Lines are inclined at an angle of 120 to each other
  4. Has both eclipsed and staggered conformations
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26
Q

Preparation of alkenes from alkynes

A

+ H2 Catalyst: Pd / C (cis isomer)
Catalyst: Na (trans isomer)

Reaction learn

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27
Q

Preparation of alkene from alkyl halide

A

Heating with alcoholic KOH (dehyrohalogenation & beta - elimination)

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28
Q

Reaction rate for alkyl and halogens

A

Alkyl : Tertiary > Sec > Pri
Halogens: Iodine > Br > Cl

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29
Q

Prep of alkene from vicinal dihalide

A

Add Zn forms ZnX2 (dehalogenation)

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30
Q

Vicinal groups

A

Groups attached to two separate carbon atoms that are adjacent

31
Q

Geminal groups

A

Groups which are attached to the same carbon atom

32
Q

Prep of alkene from alcohols

A

Acidic dehydration (add conc. H2SO4) gives water
Beta elimination

33
Q

Why does cis isomer have higher BP

A

Cis has higher BP due to more polar nature leading to stronger intermolecular dipole-dipole interaction, this requiring more heat energy to separate them

34
Q

Why is benzene so stable?

A
  1. Delocalization of pi electrons
  2. Presence of resonance
35
Q

Increase in Size increases

A

BP

36
Q

Addition of halogens to alkenes

A

Form vicinal dihalides (bromine is discharged) - unsaturation

37
Q

Addition of hydrogen halides

A

Form alkyl halides
Order of reactivity is HI>HBr > HCl

38
Q

Explain Markovnikov mechanism

A
  • Either form 2-bromopropane or 1-bromopropane
  • 2-bromopropane has more stable secondary carbocation
39
Q

What is markovnikov rule

A

States that the negative part of the addendum gets attached to that carbon atom which hass lesser hydrogen atoms

40
Q

w

Preparing alkanes from unsaturated hydrocarbons

A

Pt / Pd / Ni used as catalysts to convert / hydrogenation

41
Q

Explain kharash / anti-markivnikov effect

A
  • With the presence of peroxide
  • Secondary free radical is more stable
42
Q

Addition of sulphuric acid to alkenes

A

Addition form alkyl hydrogen sulphate (breaking double bond with OSOOOH)

43
Q

Addition of water to alkene

A

In presence of conc. H2SO4 form alcohols

44
Q

Oxidation of alkenes

A

React with cold, dilute KMNO4 form vicinal gylocols
Used for unsaturation

45
Q

Arrange the halogens F2, Cl2, Br2 and I2 in order of their increasing reactivity with alkanes

A

F2 > Cl2 > Br2 > I2 (decrease in electronegativity down the group)

46
Q

Explain ozonolysis of alkenes

A
  • Forms ozonide and then cleavage of the ozonide by Zn- H2O to smaller molecules
47
Q

Polymerization of alkenes

A

Combination of large number of ethene molecules at high pressure, high temperature and in presence of a catalyst

48
Q

Forming alkyness from vicinal dihalides

A

Undergo treatment with alcoholic potassium hydroxide to undergo dehydrohalogenation

49
Q

Preparation of alkyne from calcium carbide

A
  • Heating quick lime with coke
  • Then heating to form calcium carbide reacting with water
50
Q

Why is ethyne more acidic than ethene

A
  • Ethyne has maximum sp character which have higher electronegativity hence attract shared pair of CH bond
51
Q

Addition of halogens to alkynes

A
  • Reddish orange colour of the solution of bromine in carbon tetrachloride is decolourised
52
Q

Addition of hydrogen halides (to alkynes)

A

Two molecules of hydrogen halides add to alkynes to form geminal dihalides

53
Q

Addition of water to alkynes

A
  • Water with dilute sulphuric acid at 333K forms carbonyl compounds
  • It forms alcohol
  • Alcohol loses the H group to form aldehyde or ketone
54
Q

Cyclic polymerization of ethyne

A

Undergoes cyclic polymerization ot 873K to form benzene

55
Q

Conditions for aromaticity

A
  1. Planarity
  2. Complete delocalization of pi electrons
  3. Presence of (4n+2) pi electronsc
56
Q

Decarboxylation of aromatic acids

A

Sodium salt of benzoic acid on heating with soda lime gives benzene

57
Q

Reduction of phenol

A

Reduced to benzene by passing its vapours over heated zinc dust

58
Q

Nitration of benzene

A

Benzene heated with Conc. HNO3 and conc. H2SO4
Forms nitrobenzene

59
Q

Halogenation of benzene

A

In presence of a lewis acid (FeCl3, FeBr3 AlCl3)

60
Q

Sulphonation

A

Replacement of a hydrogen atom by sulphonic acid
Heating benzne with sulphuric acid

61
Q

Friedel-Crafts alkylation

A

Benzene with alkyl halide in presence of anhydrous AlCl3

62
Q

Friedel crafts acylation

A

Reaction of benzene with acyl halide in presence of AlCl3

63
Q

Why do alkenes undergo electrophilic addition while arenes undergo substitution?

A
  • 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
64
Q

Why is staggered conformation more free?

A
  • Staggered has minimum repulsive forces
  • Minimum energy and maximum stability
  • Magnitude of torsional strain is much lesser
65
Q

Ortho and para directing groups

A
  1. Electron density is more on o & p positiions
  2. -I effect is slightly reduced
  3. However, overall electron density increases due to resonance
  4. Electrophiles attack
66
Q

What are electrophiles

A
  • Accepts electrons (positively or neutral)
  • Electron deficient; lewis acids
  • Eg: -OH, -NH2, -CH3, -OCH3
67
Q

Nucleophiles

A
  • Donates electrons (negative or neutral)
  • Electron rich; lewis bases
  • Eg: -NO2, -CN, -CHO, -COR
68
Q

Meta directing groups

A
  • Nitro group reduces electron due to strong -I effect
  • Overall density decreases making further substitution difficult
69
Q

+I vs -I effect

A
  • Electron density is more towards the electronegative atom
  • +I involves pulling the electron density causing polarization - like Cl, F, Br
70
Q

How to determine reactivity towards an electrophile (Benzene and substit)

A

-I -R = low reactivity (OCH3)
-I +R / no R = mid (Cl)
+I +R = high reactivity (NO2)

71
Q

Why are haloarenes ortho para directing

A

-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

72
Q

How to convert benzene into p-nitrobenzene

A
  • Benzene + Br2
  • Add conc. HNO3 for nitrogen
  • Bromine is an ortho-para directing group where as NO2 is a meta-directing group
73
Q

Why does benzene undergo electrophilic substitutions easier than additions?

A

Due to presence of 6pi electrons, behaves as a rich source of electrons thus being easily attacked by reagents deficient in electrons

Chem - Gr11 (8 decks)

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