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Chem - Gr11 > Hydrocarbons > Flashcards

Hydrocarbons Flashcards

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
Sawhorse projections
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
26
Preparation of alkenes from alkynes
+ H2 Catalyst: Pd / C (cis isomer) Catalyst: Na (trans isomer) | Reaction learn
27
Preparation of alkene from alkyl halide
Heating with alcoholic KOH (dehyrohalogenation & beta - elimination)
28
Reaction rate for alkyl and halogens
Alkyl : Tertiary > Sec > Pri Halogens: Iodine > Br > Cl
29
Prep of alkene from vicinal dihalide
Add Zn forms ZnX2 (dehalogenation)
30
Vicinal groups
Groups attached to two separate carbon atoms that are adjacent
31
Geminal groups
Groups which are attached to the same carbon atom
32
Prep of alkene from alcohols
Acidic dehydration (add conc. H2SO4) gives water Beta elimination
33
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
34
Why is benzene so stable?
1. Delocalization of pi electrons 2. Presence of resonance
35
Increase in Size increases
BP
36
Addition of halogens to alkenes
Form vicinal dihalides (bromine is discharged) - unsaturation
37
Addition of hydrogen halides
Form alkyl halides Order of reactivity is HI>HBr > HCl
38
Explain Markovnikov mechanism
* Either form 2-bromopropane or 1-bromopropane * 2-bromopropane has more stable secondary carbocation
39
What is markovnikov rule
States that the negative part of the addendum gets attached to that carbon atom which hass lesser hydrogen atoms
40
# w Preparing alkanes from unsaturated hydrocarbons
Pt / Pd / Ni used as catalysts to convert / hydrogenation
41
Explain kharash / anti-markivnikov effect
* With the presence of peroxide * Secondary free radical is more stable
42
Addition of sulphuric acid to alkenes
Addition form alkyl hydrogen sulphate (breaking double bond with OSOOOH)
43
Addition of water to alkene
In presence of conc. H2SO4 form alcohols
44
Oxidation of alkenes
React with cold, dilute KMNO4 form vicinal gylocols Used for unsaturation
45
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)
46
Explain ozonolysis of alkenes
* Forms ozonide and then cleavage of the ozonide by Zn- H2O to smaller molecules
47
Polymerization of alkenes
Combination of large number of ethene molecules at high pressure, high temperature and in presence of a catalyst
48
Forming alkyness from vicinal dihalides
Undergo treatment with alcoholic potassium hydroxide to undergo dehydrohalogenation
49
Preparation of alkyne from calcium carbide
* Heating quick lime with coke * Then heating to form calcium carbide reacting with water
50
Why is ethyne more acidic than ethene
* Ethyne has maximum sp character which have higher electronegativity hence attract shared pair of CH bond
51
Addition of halogens to alkynes
* Reddish orange colour of the solution of bromine in carbon tetrachloride is decolourised
52
Addition of hydrogen halides (to alkynes)
Two molecules of hydrogen halides add to alkynes to form geminal dihalides
53
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
54
Cyclic polymerization of ethyne
Undergoes cyclic polymerization ot 873K to form benzene
55
Conditions for aromaticity
1. Planarity 2. Complete delocalization of pi electrons 3. Presence of (4n+2) pi electronsc
56
Decarboxylation of aromatic acids
Sodium salt of benzoic acid on heating with soda lime gives benzene
57
Reduction of phenol
Reduced to benzene by passing its vapours over heated zinc dust
58
Nitration of benzene
Benzene heated with Conc. HNO3 and conc. H2SO4 Forms nitrobenzene
59
Halogenation of benzene
In presence of a lewis acid (FeCl3, FeBr3 AlCl3)
60
Sulphonation
Replacement of a hydrogen atom by sulphonic acid Heating benzne with sulphuric acid
61
Friedel-Crafts alkylation
Benzene with alkyl halide in presence of anhydrous AlCl3
62
Friedel crafts acylation
Reaction of benzene with acyl halide in presence of AlCl3
63
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
64
Why is staggered conformation more free?
* Staggered has minimum repulsive forces * Minimum energy and maximum stability * Magnitude of torsional strain is much lesser
65
Ortho and para directing groups
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
What are electrophiles
* Accepts electrons (positively or neutral) * Electron deficient; lewis acids * Eg: -OH, -NH2, -CH3, -OCH3
67
Nucleophiles
* Donates electrons (negative or neutral) * Electron rich; lewis bases * Eg: -NO2, -CN, -CHO, -COR
68
Meta directing groups
* Nitro group reduces electron due to strong -I effect * Overall density decreases making further substitution difficult
69
+I vs -I effect
* Electron density is more towards the electronegative atom * +I involves pulling the electron density causing polarization - like Cl, F, Br
70
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)
71
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
72
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
73
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

Chem - Gr11 (8 decks)

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