Bromination Lab (Midterm)

Typical alkenes are __-__.

Electron-rich

Typical alkenes react with __.

Electrophiles

Addition reaction:

Bimolecular. Rate dependent on both alkyl halide and base concentrations

Vicinal dibromide

Elimination (E2)

Alkyne

Electrophilic addition without electron withdrawing groups

H-Br as electrophile

In addition of H-Br to alkene, the stereochem __ if it is __.

Inverts, Chiral

Addition reaction:

Atoms of both reactants are combined in the product

Alkene grabs one Bromine as bromine grabs alkene

Triangle with +Bromine attached to both alkene carbons (Bromonium ion)

The reaction with Br2 creates a __ ion with a __ charge and not a carbocation.

Bromonium. Positive charge

Bromonium ion is ___ and reacts with nucleophiles in ___ ___ to open the ring.

Stereospecific. Stereospecific SN2 (backside attack)

Double dehydrohalogenation

Vicinal dibromide from addition reaction

Double dehydrohalogenation

An alkenyl halide (alkene with a halide)

Starting material for our addition

Trans-stilbene

Intermediate alkane from addition before elimination

Dibromide (meso-stilbene dibromide)

Double elimination product

Diphenylacetylene (symmetrical alkyne)

In bromination and dehydrohalogenation we used solid compound __ __ __ (__), as source of bromine, which avoids hazards of Br₂.

Pyridinium hydrobromide perbromide (PyHBr₃)

The elemental form of Br₂ wasn’t used because it is a __ __ __.

Volatile liquid oxidant

PyHBr₃ releases one molar equivalent of __ so that __ __ reactions can occur.

Br₂. Electrophilic addition

Green chemistry principles used:

Low toxicity - solid form of Br₂ to minimize vapors of Br₂

We performed a __-induced double __ reaction of a vicinal dihalide.

Base-induced double elimination

Carbons of alkenes…

Sp2 hybridized with leftover p-orbital with one electron

Leftover p-orbitals of two carbons can combine to make…

New molecular orbitals / HOMO and LUMO

The reaction would occur more readily with the racemic chiral dibromide since the leaving group (Br) and hydrogen are ___. For an E2 elimination ___ is more favorable which makes occur readily.

Antiperiplanar (x2)

Bromonium ion product relationship

Diastereomers. Stereochem around alkene Carbons is retained/stereospecific (cis-cis, trans-trans)

Triple bond of an alkyne appears in IR spectrum for ___ alkynes.

Unsymmetrical

The triple bond frequency in IR is _____.

2100-2260 cm^-1

Strong __ initiate dehydrohalogenation. Concerted reactions __ depend on __ __.

Bases. E2 bimolecular reactions depend on antiperiplanar hydrogens

The diphenylacetylene’s alkyne stretch is __ expected to show up on the IR spectrum.

Not expected