Preparation of 2,2,6-trimethyl-5-phenylheptan-3-one:

Safety

2-Bromopropane : Irritating to eyes, respiratory system and skin. If eye contact, wash with large volumes of water and seek medical help.Flammable. Avoid breathing vapour.

Magnesium : Contact with acid produces hydrogen which is flammable.

Ether : Highly flammable - segregate from all ignition sources. Use new stock only, as old material may contain explosive peroxides. Irritating to eyes, respiratory system and skin.

Stage 1 product : No data available - treat as irritant.

Stage 2 product : No data available - treat as irritant.

Comments

Since Grignard reagents are destroyed instantaneously by water (and other protic compounds), all apparatus and materials must be scrupulously dry.

• Dry glassware for 45 min in an oven at 120o.
• Dry Grignard-grade magnesium turnings for 30 min in an oven at 100o.
• Dry 2-bromopropane overnight over anhydrous magnesium sulphate.
• Use diethyl ether which has been dried overnight over sodium wire.

All stages in the preparation and use of Grignard reagents are exothermic, so have an ice-water slush-bath to hand throughout.

Do not plan to store your Grignard solution: use it as quickly as possible.

Procedure

(a) Preparation of 2-propyl magnesium bromide

Beware: If your apparatus is only slightly damp the reaction will take a very long time to complete

Equip a 3-neck 250 mL RB flask with a magnetic flea, a stopper, a Liebig condenser surmounted by a calcium chloride drying tube, and a stoppered dropping funnel. Mount the apparatus on a retort stand, on the base-plate of which stands a partially jacked up lab-jack, supporting a heater/magnetic stirrer unit. (Note 9).

To the flask, add the dry magnesium (1.08 g, 0.045 moles), and enough dry ether to cover it (ca. 10 mL). Place a solution of 2-bromopropane (isopropyl bromide; 5.54 g, 0.045 moles) in dry ether (35 mL) in the dropping funnel. Add a few mLs of this solution rapidly to the magnesium, at ambient temperature, and allow reaction to proceed.

If nothing happens during the next 5 min, use a clean, dry glass-rod (through the stoppered opening of the flask) to scrape firmly some of the magnesium against the sides of the flask, beneath the surface of the ether (Note 10). The onset of reaction is indicated by the occasional bubble rising from the magnesium to the surface: this will soon become a stream of bubbles. The temperature of the solution will rise, and the solution will begin to reflux spontaneously. At this point, start the stirrer, and slowly add the remainder of the halide solution to maintain self-sustaining, but controlled, reflux. (Do not cool with your slush-bath unless the reaction appears to be getting out of control.)

When addition of the halide is complete, and the reflux slackens, stir and heat under reflux for a further 15 min, until almost all of the magnesium has been used up (Note 11). You should now have a solution of 2-propyl magnesium bromide containing, perhaps, a few darkish particles of unreacted magnesium.