Convert each compound to a Fischer projection and label each stereogenic center as R or S.

(a.)

(b.)

(c.)

(d.)

(e.)

(f.)

Fischer projection is the 2D representation and shows an absolute configuration with chiral centers. Horizontal lines describe bonds in the forward direction, and vertical lines represent bonds in the rear direction.

By rotating and redrawing the compounds, the horizontal bonds come in front of the plane, and vertical lines come behind the plane.

a. ${\mathrm{CH}}_3$and Br are bonds placed in front of the plane, so they come at a horizontal line. H and ${\mathrm{CH}}_3\mathrm{COOH}$are bonds placed out of the plane, placed at the vertical line. The stereogenic center has an S configuration.

Fischer projection and stereogenic center of ab.${\mathrm{CH}}_3\mathrm{and}{\mathrm{CH}}_2{\mathrm{CH}}_3$bonds are in the plane to be placed at a horizontal line. ${\mathrm{CH}}_3\mathrm{O}\mathrm{and}{\mathrm{OCH}}_2{\mathrm{CH}}_3$bonds are below the plane, so they are vertical. This compound is redrawn to get the exact configuration. The stereogenic center has an S configuration.

Fischer projection and stereogenic center of b

Fischer projection and stereogenic center of cd. H and Br bonds of C1 and H and Cl bonds of C2 are in the plane, so they are placed at the horizontal line. ${\mathrm{CH}}_3\mathrm{and}{\mathrm{CH}}_2{\mathrm{CH}}_3$bonds are below the plane, so they are vertical. The stereogenic centers C1 and C2 have S configuration.

Fischer projection and stereogenic center of d

Fischer projection and stereogenic center of ef. The compound is redrawn to get the Fischer projection. C1 and C2 have S configurations, and C3 has R configuration.

Fischer projection and stereogenic center of f