haloalkanes and haloarenes Flashcards
nomenclature, preparation of haloalkanes and haloarenes, reactions with both, physical properties, polyhalogen compounds (73 cards)
1
Q
chloramphenicol
A
- antibiotic
-produced by microorganisms in soil
-treats typhoid fever
2
Q
halothane
A
- dental anaesthetic during surgery
3
Q
Chloroquine
A
- synthetic compound used to treat malaria
4
Q
thyroxine
A
- iodine containing hormone found in thyroid gland, iodine deficiency causes goitre
5
Q
perfluorinated compounds
A
- potential blood substituents in surgery
6
Q
alkyl halides
A
halogen atom bonded with sp3 hybridised carbon atom
CnH2n+1X
7
Q
Allylic halides
A
attached to allylic carbon that is, carbon atom adjacent to a carbon-carbon double bond
8
Q
Benzylic halides
A
halogen atom attached to carbon which is attached to aromatic ring
9
Q
Vinylic halides
A
halogen atom attached to sp2 hybridised carbon atom
10
Q
Aryl halides
A
dirrectly attached to sp2 carbon atom of an aromatic ring
11
Q
alkylidene halides
A
- gem dihalides
- a single carbon atom has 2 halogen atoms attached
12
Q
alkylene halides
A
- vicinal dihalides
- 1 halogen present on adjacent carbon atoms each
13
Q
3 ways to prepare haloalkanes from alcohols?
A
1) with halogen acids
2) phosphorous halides
3) thionyl chloride
14
Q
why is thionyl chloride preferred? formula?
A
because the byproducts- SO2 and HCl gas are escapable hence, the reaction gives pure alkyl halides. SOCl2.
15
Q
how are preparation of primary and secondary haloalkanes diff from tertiary?
A
primary and secondary require presence of a catalyst called ZnCl2 whereas, tertiary alchohols can simply be shaken with conc. HCl at room temp.
16
Q
what does ZnCl2 do?
A
coordinates with O atom of OH group thus weakening C-O bond and breaking it to form a carbocation
17
Q
How are PBr3 and PI3 prepared as compared to PCL3 and PCL5
A
since PBr3 and PI3 are unstable compounds, they are prepared ‘in situ’, reaction of red phosphorous with either Br2 or I2 can form R-X2
18
Q
in which 2 ways can alkyl bromides be prepared?
A
1) refluxing/constant boiling of alcohol with 48% HBr in presence of conc.H2SO4 to give R-Br and H2O
2) heating alcohol with sodium bromide and H2SO4 to give R-Br in situ and NaHSO4
19
Q
in which 2 ways can Iodides be prepared
A
1) refluxing with HI in presence of conc. H2SO4 to give R-I and H2O
2) heating alcohol with sodium or potassium iodide in 95% orthophosphoric acid to give RI + NaH2PO4 + H2O
20
Q
all the ways in which haloalkanes can be prepared from hydrocarbons?
A
1) free radical halogenation to alkanes
2) addition of hydrogen halides to alkenes
3) addition of halogens to alkenes
4) allylic halogenation
21
Q
what is the test for unsaturation
A
addition of bromide in CCl4 to an alkene resulting discharge of reddish brown colour, indicating double bond
22
Q
allylic halogenation
A
substitution of H atom on an allylic carbon by a halogen atom at 773 K
23
Q
finkelstein
A
reaction of alkyl bromides/ chlorides with Sodium Iodide (NaI) in dry acetone which gives alkyl iodide and sodium bromide/chloride
24
Q
what is the benefit of finkelstein?
A
Sodium bromide or chloride formed is insoluble in dry acetone and is thus precipitated, this can be easily removed from the reaction mixture which facilitates the forward reaction according to le chatelier’s principle.
25
swarts reaction
by treating alkyl bromides/chlorides with metallic fluorides such as AgF, Hg2F2, CoF2, SbF3 which produce alkyl fluoride and corresponding metallic halide.
26
how to do fluorination in free radical halogenatuon?
it is too violent but can be done if fluorine is diluted with an inert gas such as argon.
27
why cant aryl halides be prepared from phenols?
because there os presence of partial double bond character in carbon-oxygen bond of phenols and is thus difficult to break as it is stronger than single bond.(DRAW RESONANCE STRUCTURE!)
28
why does iodination require an oxidising agent?
because iodination is a reversible reaction and hence, requires an oxidising agent such as nitric acid or HIO3
29
why cant fluoro compounds be prepared using electrophilic substitution
fluorine has high reactivity
30
how to prepare aryl halide from diazonium salts in case of bromine and chlorine? what is sandmeyer reaction?
1) primary, aromatic amine dissolved or suspended in cold, aqueous, mineral acid, is treated with sodium nitrite and hydrogen chloride/bromide at 0-5 C to produce benzyl diazonium halide.
2) the diazonium chloride/bromide is treated with cuprous chloride or bromide which forms the aryl halide and N2
-> Sandmeyer reaction is when the diazonium salt is treated with cuprous halide to form the corresponding aryl halide.
31
why is free radical halogenation not useful?
it produces a complex mixture of mono-and poly-haloalkanes whose boiling points are too close and therefore, too difficult to separate
32
in presence of what do u do free radical halogenation?
Uv light or heat
33
GATTERMANN REACTION?
when diazonium salt is treated with copper powder instead of cuprous halideT
34
how to prepare iodine from diazonium salt?
shaking with potassium iodide
35
BALZ-SCHIEMANN REACTION?
producing fluoroarene by treating the aromatic amine with borofluoric acid (HBF4) which produces benzene diazonium tertafluoroborate which on heating, gives fluorobenze, boron trifluoride and N2
36
why is sulphuric acid not used during the reduction of alcohols with KI?
it cannot be used along with KI in the conversion of an alcohol to an alkyl iodide as it converts KI to corresponding acid, HI, which is then oxidised by it into I2.
37
why do alkyl halides have greater boiling points than their parent hydrocarbons?
because of greater molecular mass of the halogens and greater van der Waals forces and dipole-dipole interactions
38
why do para isomers of dihalobenzenes have higher melting point?
due to their symmetry which allows them to pack well in the crystal lattice
39
why are haloalkanes insoluble in water?
Because the energy released during hte set up of the new attractiions between the haloalkane and the water molecules is lesser than that which is required to overcome the attractions between haloalkane molecules and break hydrogen bonds in water.
40
what are ambident nucleophiles and examples?
groups with 2 nucleophilic centres such as nitrites and cyanides.
- cyanides are hybrids of 2 contributing structures in whch linking thru carbon atom resulkts in formation of alkyl cyanides and through nitrogen atom results in formation of isocyanides.
- similarly, in nitrites, linking through oxygen atom results in alkyl nitrites and through linking of nitrogen atom results in nitroalkanes
41
KCN forms alkyl cyanides and AgCN forms isocyanides
-KCN is predominantly ionic and provides cyanide ions in solution and although both carbon and nitrogen are free to donate, C-C bond is more stable than C-N bond whereas, in case of AgCN, it is mainly covalent in nature and nitrogen is free to donate electron pair thus forming isocyanide as main product
42
what order kinetics do substituition nucleophilic bimolecular follow?
second order kinetics which means, rate of reactions depend on concentration of both the reactants
43
walden inversion?
configuration is inverted and this is known as walden inversion.
44
what solvents are unimolecular nucleophilic substitutions carried out in and what order kinetics?
polar, protic solvents such as water, alcohol, or acetic acid.
they follow first order kinetics which means rate of reaction depends only on concentrastion of one reactant, that is, the alkyl halide as the slowest step is breaking of the C-Br bond in the alkyl halide.
45
order of stability for Sn1 and Sn2 reactions
Sn2: tertiarysecondary>primary>methyl halides (due to greater stability of tertiary carbocation)
46
DEFINE RACEMISATION
THE PROCESS OF CONVERSION OF AN ENANTIOMER INTO A RACEMIC MIXTURE
47
DEFINE RACEMIC MIXTURE
a mixture containing 2 enantiomers in equal proportions which will have zero optical rotation as the rotation due to one isomer will be cancelled by the rotation due to the other.
48
what is retention of configuration?
if during a reaction, no bond to the stereocentre is broken, the product will have the same general configuration of groups around the stereocentre of the reactant.
49
WHY ARE SN1 REACTIONS ACCOMPANIED BY RACEMISATION OF OPTICALLY ACTIVE ALKYL HALIDES?
BECAUSE THE CARBOCATION FORMED IS PLANAR DUE TO SP2 HYBRIDISATION OF THE CARBOCATION THUS THE ATTACK OF NUCLEOPHILE MAY BE ACCOMPLISHED FROM EITHER SIDE OF THE PLANE RESULTING IN A MIXTURE OF PRODUCTS, ONE HAVING SAME CONFIGURATION AND OTHER HAVING INVERTED.
50
what are elimination reactions
when a haloalkane with B-hydrogen atom is heated with alcoholic solution of potassium hydroxide, there is elimination of hydrogen from B-carbon and halogen atom from alpha carbon and as a result, an alkene is formed.
51
STATE ZAITSEV/SAYTZEFF RULE
IN DEHYDROHALOGENATION REACTIONS, THE PREFERRED PRODUCT IS THE ALKENED WHICH HAS GREATER NUMBER OF ALKYL GROUPS ATTACHED TO THE DOUBLY BONDED CARBON ATOMS.
52
WHAT ARE GRIGNARD REAGENTS?
alkyl magnesium halides which are obtained by reactions of haloalkanes with magnesium metal IN DRY ETHER.
53
why is it necessary to avoid traces of moisture from a grignard reagent/why is it carried out in dry ether
grignard reagents are highly reactive and even water, amines, and alchohols are sufficiently acidic to convert them into corresponding hydrocarbons.
RMgX + H2O-dry ether-> R-H + Mg(OH)x
54
why are haloarenes extremely less reactive to nucleophilic substitution reactions?
1. The electron pairs of the halogen atom are in conjugation with pi electrons of the ring. C-Cl acquires a partial double bond character due to resonance and hence, bond cleavage of C-Cl bond is more difficult than in haloalkanes.
2. The halogen atom is attached to a sp2 hybridised carbon in haloarenes and to an sp3 hybridised carbon in haloalkanes. An sp2 carbon having greater s character is more electronegative and can hold the electron pair tightly as compared to the sp3 hybridised carbon. C-Cl bond lenght in haloalkanes is 177 pm wheres in haloarenes it is 169 pm and since it is difficult to break a shorter bond, haloarenes are less reactive to nucleophilic substitution.
3. The phenyl cation formed as a result of self-ionisation will not be stabilised by resonance, therefore Sn1 mechanism is ruled out.
4. Due to POSSIBLE REPULSIONS, it is less likely for an electron rich nucleophile to approach electron rich arenes.
55
DOW'S PROCESS/ how can chlorobenzene be converted to phenol?
i) heating in aqueous sodium hydroxide solution at 623 K and 300 atm which results in formation of sodium phenoxide.
ii) H+ is added and phenol is formed
56
conditions for heating nitrobenzenes to nitrophenols
1) for 1 NO2 grp: NaOH, 443K, H+
2) for 2 NO2 grps: NaOH, 368K, H+
3) for 3 NO2 grps: warm H2O
57
why are electrophilic substitution reactions slower in haloarenes as compared to benzene?
although the electron density increases at ortho and para positions, the ring gets deactivated due to I- effect of the halogen atom due to its tendency to withdraw electrons towards itself hence, E sub reactions are slower and require more drastic conditions as compared ot benzene.
58
reagents for EVERY electrophilic substitution reactions
1) halogenation: Anhydrous FeCl3
2) Nitration: nitrating mixture consisting of HNO3 and conc.H2SO4
3) sulphonation: conc.H2SO4
4) Friedel-Crafts alkylation and acylation both use anhydrous. AlCl3
59
names of CH3COCl
1) Acetyl chloride
2) acetic anhydride
3) ethanoyl chloride
60
Fittig reaction
2 aryl groups treated with sodium in dry ether in which they are joined together
61
how to convert ethanol to but-1-yne
i) treat ethanol with SOCl2 which forms ethyl chloride
ii) treat ethyl chloride with sodium acetylide/sodium ethynide (CH- - -C-Na+)
62
convert propene to propyne
i) bromination in carbon tetrachloride
ii) dehydrohalogention using alc koh
iii) treating with NaNH2 in liquid NH3
63
convert bromomethane to propanone
i) treat with potassium cyanide
ii) treat CH3CN with CH3MgBr- [(CH3)2-C=NMgBr]
iii) acid hydrolysis (H+/H2O)- propanone
64
convert isopropyl alcohol to iodoform
i) treat with I2/NaOH- Iodoform test givrs CHI3 AND CH3COONa
65
benzene to 4-bromonitrobenzene
i) add Br2 in presence of Fe and Nitration
66
list polyhalogen compounds
1) Dichloromethane/Methylene chloride
2) Trichloromethane/Chloroform
3)Triiodomethane/Iodoform
4)Tetracloromethane/Carbon tetrachloride
5) Freons
6) p,p'-Dichlorodiphenyltrichloroethane (DDT)
67
explain dichloromethane
- propellant in aerosols, paint remover, solvent, metal cleaning
- harms human CNS
- exposure to lower levels of methylene chloride can lead to slightly impaired hearing and vision.
- higher levels- dizziness, nausea, tingling, numbness in fingers and toes.
- direct skin contact- intense burning, mild redness, can burn the cornea
68
trichloromethane
- production of freon refrigerant R-22
- inhaling chloroform depresses the CNS, breathing 900 ppm can cause fatigue, dizziness, and headache
- slowly oxidised by air in the presence of light to form an extremely poisonous gas, carbonyl chloride aka phosgene (which is also formed by metabolising chloroform in the liver causing damage and to kidneys)
- this is why its kept in close dark coloured bottles completely filled without air
WRITE EQUATION
69
FORMULA OF PHOSGENE
2COCl2
70
iodoform
used as an antiseptic due to liberation of free iodine but due to its objectionable smell it has been replaced by other iodine
71
tetracloromethane
-synthesis of CFC's, general solvent use, pharmaceutical manufacturing, cleaning fluid, spot remover, fire extinguisher.
- causes liver cacner in humans
-dizziness, light headedness, vomiting
stupor, coma, unconsciousness, death
- irregular heart beat or stop.
-depletes ozone layer
72
how are freons manufactured
from tetrachloromethane by swarts reaction
upset natural ozone balance
73
aniline to phenylisocyanide
aniline + CHCl3+ 3KOH ------> (carbylamine)---> phenylisocyanide + 3 KCl+3H2O
