Two steps:

1. Form the Grignard reagent with 1-bromopropane and magnesium metal. This can be down in a variety of ethereal solvents, THF or $\ce{Et2O}$ are most commonly used. This species is nucleophilic through carbon and will add to the carbonyl group.

 Cool the Grignard solution in an ice bath under nitrogen with stirring, slowly add a solution of 0.9 eq of benzophenone. Allow it to stir until TLC shows consumption of the benzophenone starting material.

 Add a solution of aqueous ammonium chloride to quench residual Grignard. Add organic solvent $(\ce{Et2O}$ or $\ce{EtOAc})$ and collect the organic phase, dry and concentrate. This will form 1,1-diphenyl-butan-1-ol.

2. Take the crude product from step 1 and dissolve in toluene. Add a catalytic amount of p-toluene sulfonic acid (PTSA) and heat under Dean-Stark reflux to remove the water. The PTSA will protonate the tertiary alcohol you have formed in step 1. This gives a cation stabilising by the two phenyl groups so it forms readily. This eliminates water to give the required product cleanly.

 When the system has ceased to generate water allow it to cool, wash with diluted aqueous sodium carbonate solution, dry and concentrate. This should result in fairly clean 1,1-diphenyl-1-butene.

 To produce cleaner material use chromatography on silica with hexane/ethyl acetate (0 to 10%).

Further reading on Grignard reagents: [Master Organic Chemistry][1].


  [1]: https://www.masterorganicchemistry.com/2011/10/14/reagent-friday-grignard-reagents/