4-Cyano-4'-pentylbiphenyl is a commonly used nematic liquid crystal with the chemical formula C<sub>18</sub>H<sub>19</sub>N. It frequently goes by the common name 5CB. 5CB was first synthesized by George William Gray, Ken Harrison, and J.A. Nash at the University of Hull in 1972 and at the time it was the first member of the cyanobiphenyls. The liquid crystal was discovered after Gray's group received a grant from the UK Ministry of Defence to find a liquid crystal that had liquid crystal phases near room temperature with the specific intention of using them in liquid crystal displays. The molecule is about 20 àlong. The liquid crystal 5CB undergoes a phase transition from a crystalline state to a nematic state at 22.5 ðC and it goes from a nematic to an isotropic state at 35.0 ðC.
5CB is produced by modifying biphenyl in a linear manner. First Br<sub>2</sub> is added to the biphenyl to introduce a bromine atom to the end of the moiety. Next aluminium chloride and C<sub>4</sub>H<sub>9</sub>COCl is added to the sample, followed by the addition of potassium hydroxide and NH<sub>2</sub>NH<sub>2</sub>. By this point the molecule will have a bromine atom on one end of the rigid core and C<sub>5</sub>H<sub>11</sub> on the other end. Finally, introduction of copper(I) cyanide and DMF results in the removal of the bromine and its replacement with CN, yielding 5CB.