W26 - Haloalkanes

Flashcards about carbon and functional group chemistry.

What are halogenoalkanes also known as?

Haloalkanes or alkyl halides

What is special about halogenoalkanes?

Adding a halogen to an alkane makes it reactive.

What is the inductive effect?

The ability of a chemical group to pull or push electrons.

How do CH3 groups affect electrons?

They 'push' electrons

How do halogens affect electrons?

They 'pull' electrons

How does methyl affect carbocation stability?

Methyl groups stabilize a carbocation.

How does chlorine affect carbocation stability?

Chlorine destabilizes a carbocation.

How does chlorine affect carbanion stability?

Chlorine stabilizes a carbanion.

List the relative reactivity of halogenoalkanes from most to least reactive.

R-I > R-Br > R-Cl >> R-F

Why is reactivity due to polarisation of the C-X bond?

Electronegative halogen attracts electrons, making the halogen δ- and carbon δ+.

What properties do nucleophiles possess?

Electron-rich, possess at least one lone pair of valence electrons, and are attracted to centres deficient in negative charge.

What makes a good nucleophile?

Conjugate bases of weak acids

What makes a good leaving group?

Conjugate bases of strong acids

What kind of leaving group is OH-?

A poor leaving group

How is alcohol halogenation performed, and why?

Usually done in acid (H+) because OH- is a poor leaving group. The acid protonates the alcohol oxygen, creating a better leaving group (H2O).

What are some other methods of forming halogenoalkanes?

Alcohol + phosphorous trihalide (PX3), Alcohol + thionyl chloride (SOCl2), Alkene + hydrogen halide (HX), Alkene + halogen (X2)

What is the golden rule for drawing reaction mechanisms?

Arrows ALWAYS show the movement of electrons NOT atoms

What do SN1 and SN2 stand for?

SN1 = Substitution, Nucleophilic, 1 molecule (unimolecular); SN2 = Substitution, Nucleophilic, 2 molecules (bimolecular)

What happens to the carbon center in SN2 reactions?

The carbon centre inverts like an umbrella turning inside out in the wind!

What is steric hinderance influence on the reaction in Nucleophilic Substitution: SN2?

When bulky groups are around the central carbon, it slows the reaction

What is Walden Inversion?

Inversion of the Carbon Centre during SN2

What type of product does SN1 create?

An equal mix of chiral products

What is a racemate or racemic product?

A 50:50 mixture of chiral products

What type of solvents does SN2 prefer?

Polar, aprotic solvents

What type of solvents does SN1 prefer?

Polar, protic solvents

When does SN1 dominate?

Many bulky ‘R’ groups, creates a more ‘stable’ intermediate carbocation C+, steric acceleration allows leaving group to ‘leave’, solvent is polar and protic

When does SN2 dominate?

Few bulky ‘R’ groups, creates a larger d+ charge for nucleophilic attack, reduction in steric hindrance that prevents SN2 nucleophilic attack, the solvent is polar and aprotic

What type of nucleophile is a base?

a nucleophile

What can E2 stereoselectivity be used to obtain?

A Z- or E- alkene

Is E1 streoselective?

No (can get equal mixtures of Z and E), but E2 is stereoselective (one of E or Z dominates)

Is E1 regioselective?

Yes: the major product is typically the most substituted alkene.

What is SN2 favored by?

Small strong nucleophiles

What is E2 favored by?

3° halogenoalkanes, large strong bases and high temperatures

What is SN1 favored by?

Small nucleophiles

What is E1 favored by?

Large bases and high temperatures

During Electrophilic Addition to Alkenes what type of halide ion can equally attack either side of the carbocation?

SN1 reaction

What product is produced by Markovnikov Addition?

Markovnikov Product

What is a Markovnikov product?

The hydrogen atom bonds to the carbon with the greater number of hydrogen atoms, and the halide (X) bonds to the carbon with the fewer hydrogen atoms.

In rearrangements on addition of HX, which is more stable: tertiary carbocations or secondary carbocations?

Tertiary carbocations

When chiral compounds make a racemate, what mechanism is occurring?

SN1

When chiral compounds invert a chiral center, what mechanism is occurring?

SN2

What type of rearrangements occur under high temperatures?

Rearrangements occur - Mix of two E isomers - 1,2 alkyl shifts - major product most substituted alkene (Regioselectivity)

What type of products dominate under high temperatures?

E alkenes dominate products