Alkyl, Aryl Halides Flashcards
Explain Nucleophilicity.
ability to attack on electron deficient atom.
Explain Basicity.
Tendency to share electron pair or nucleophilicity towards H+.
Leaving group ability α
1/ Basic strength
Decreasing order of some LG compounds
-N⁺⩧N ≻ Triflate ≻ Brosylate ≻ Tosylate ≻ Mesylate ≻ I⁻ ≻ Br⁻ ≻ Cl⁻ ≻ F⁻ ≻ Carboxylic acid ≻ RO⁻ ≻ OH⁻
Rules for nucleophilicity and basicity
- Across the period both nucleo. and basicity decreases.
- If donor atom is same, then nucleophilicity α basicity.
- Nucleophilicity α 1/steric hindrance
- Anions are stronger Nu: than neutral molecules.
- Down the group depends on solvent.
protic- Nu: decreases down the group
aprotic - Nu: increases down the group
Name some Polar aprotic solvents.
DMSO (Dimethyl Sulfoxide), Diethyl ether, DMF (Dimethyl formamide), DMA (Dimethylacetamide), THF (Tetrahydrofuran), DME (Dimethoxy ethane), HMPA (Hexamethyl Phosphoric amide)
Effects of Nu: in polar protic and polar aprotic solvents
In polar protic solvents, Nu: is stabilized hence tendency to attack decreases. F⁻ ≺ Cl ⁻ ≺ Br⁻ ≺ I⁻
In aprotic, Nu: is not stabilized = Nu: α charge/radius.
F⁻ ≻ Cl⁻ ≻ Br⁻ ≻ I⁻
The major mechanism depends upon-
i) structure of RX
ii) strength of Nu:
iii) concn. of Nu:
iv) nature of solvent
SN1 is favoured by
- Polar protic solvents; they help in dissociation of C-X bond.
- 3°RX≻2°RX≻1°RX≻CH3-X
- R-I≻ R-Br≻ R-Cl ≻R-F (LG ability)
- low concentration of weak Nu:
What is solvolysis?
Reactions in which solvent acts as Nu:
Stereochemistry of SN1 reaction:*
- The dissociation of C-X bond in an SN1 reaction leads to the formation of intimate ion-pair (C⁺X⁻)
- Thus, there is some steric hindrance on the front side, i.e., on the leaving group. This makes backside attack more feasible.
- The extent of inversion in an SN1 reaction is found to be 50% - 70%. Therefore, SN1 reaction occurs with the partial racemization, and the product is generally NOT optically inactive (if it chiral).
- Extent of racemization α carbocation stability.
- SN1 reactions are favored by low concentration of weak Nu: (C⁺ is unstable it can react with any Nu:) and polar protic solvents.
Rate of SN1 and SN2 -
k [R-X] and k [R-X] [Nu:] respectively
Why does the Nu: attacks from backside in SN2?
because of the steric hindrance.
there is a 100%racemisation and a transition state where the LG is leaving and Nu: is attaching.
How does the reactivity depend in SN1 and SN2?
SN1 reactivity α carbocation stability and SN2 reactivity α 1/steric hindrance.
Explain stereochemistry of SN2.
- The product of a SN2 reaction is enantiomerically pure, i.e., is a pure enantiomer, if the product is chiral.
- The inversion of configuration in an SN2 reaction indicates that the carbon under nucleophilic attack has undergone an inversion of configuration (Waldon inversion).
- favored by high concentration of strong Nu: and polar aprotic solvents.
- An SN2 reaction is reversible if the leaving group
and the nucleophile have similar basic strengths. This is because a strong base can replace a weak base but not vice versa.
If there is a π bond or lp on the atom adjacent to the carbon atom under attack, the transition state is resonance stabilized, and such molecules react via
SN2 meachanism
Why vinyl halides and aryl halides are extremely less reactive towards SN1 and SN2 reactions because:
1.The departure of X⁻ becomes difficult due to the greater EN (sp2) of the carbon atom and the partial D.B nature of C-X bond.
2. SN1 Mechanism not feasible due to highly unstable carbocation.
3. SN2 mechanism not feasible because of e⁻-e⁻ repulsions.
Note: If the departing halogen is at the bridgehead carbon, SN1 and SN2 are not feasible due to non-planar carbocation and steric hindrance on the backside respectively.
How does the bp depends on the compound?
It is directly proportional to the molar mass and indirectly proportional to the branching.
Explain briefly About Iodoform with its chemical formula.
CHI3:
*Acts as an antiseptic due to the liberation of free Iodine.
*Due to its objectionable smell, it has been replaced by other antiseptics containing iodine.
Explain briefly uses and effects of Dichloromethane with its chemical formula.
CH2Cl2: Methylene chloride
USES:
* as a paint remover, propellent in aerosols and as a metal cleaning and finishing solvent.
Effects:
*Harmful for human central nervous system
*Causes impaired hearing and vision in low concentration.
*causes dizziness, nausea, tingling and numbness in the fingers and toes in high concentrations.
*Direct contact causes with the eyes can burn the cornea.
Chloroform
CHCl3:
USES
*As a solvent for fats, alkaloids and iodine.
*In making freon refrigerant R-22
*Earlier used as a general anesthetic (now replaced by ether)
EFFECTS:
*Inhaling chloroform vapors depresses the central nervous system.
*Causes dizziness, fatigue and headache at 900ppm.
*Chronic exposure can damage the liver (due to oxidation of CHCl3 to COCl2), kidneys, and causes sores in skin.
Stored in closed, filled dark-colored bottles, as CHCl3 in slowly oxidized to a poisonous gas phosgene, COCl2. In presence of light.
Tetrachloromethane
CCl4:
USES:
*In making refrigerant and propellants for aerosol cans.
*As feedstock in making CFCs
Effects
*causes liver cancer in humans.
*causes dizziness, light-headedness, nausea and vomiting, which can permanently damage nerve cells.
*causes stupor, coma, unconsciousness or death in some cases.
*can affect/stop heart beat.
*Acts as an eye irritant.
*Responsible for the depletion of ozone layer.
Freons
*Chlorofluorocarbon compounds of methane and ethane.
*Extremely unreactive, non-toxic, non-corrosive and easily liquefiable.
*Freon-12 (CCl2F2): made via Swarts reaction.
*Responsible for the depletion of ozone layer.
