Use Newman and other stereochemical projections

Wedge–dash, sawhorse and Newman drawings are different two-dimensional ways to represent a three-dimensional molecule.

GCE A-Level H3 Chemistry 9813-2027

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Learning objectives

Use Newman and other stereochemical projections

Choose a projection that shows the needed geometry

Wedge–dash, sawhorse and Newman drawings are different two-dimensional ways to represent a three-dimensional molecule.

A Newman projection looks directly along a specified carbon–carbon bond; a sawhorse shows the same bond obliquely; wedge–dash notation shows bonds in the page, towards the viewer and behind the page.

Projection conventions

In wedge–dash notation, an ordinary line lies in the page, a solid wedge points towards the viewer and a hashed wedge points behind the page. In a sawhorse drawing, the C–C bond is drawn diagonally and each carbon keeps its own three substituents.

In a Newman projection, the front carbon is a point with three bonds and the rear carbon is a circle with bonds beginning at its rim. Staggered front and rear bonds are offset by 60°, whereas eclipsed bonds align at 0°.

Preserve structure while changing the view

Before converting any projection, label the atoms and copy the three substituents belonging to each carbon. Then fix the viewing direction and transfer relative positions; never move a substituent from one carbon to another.

A sawhorse and Newman drawing of the same sight line must preserve every dihedral relationship. A wedge–dash conversion must preserve which bonds point towards and behind the viewer, even if the whole molecule is rotated to make the drawing clearer.

Changing a dihedral angle creates a different conformation; rotating the complete drawing or molecule in space does not change its stereochemical identity.

A useful way to choose a drawing is to ask what the question needs you to compare. Use wedge–dash to show a tetrahedral centre, Newman to compare groups along a C–C bond, and a chair to decide axial, equatorial, up and down positions in cyclohexane.

Build anti-butane

View butane from C2 towards C3, so C2 is the point and C3 the circle.

Place one methyl on each carbon 180° apart and offset all rear bonds to obtain staggered anti-butane.

Try this

Draw anti-butane viewed C2→C3.

Check your answer

A strong answer gives front/rear ownership, staggered bonds and a 180° methyl–methyl dihedral.

Convert a sawhorse into a Newman projection

Supplied drawing: an anti-butane sawhorse viewed from C2 towards C3.

Decide which carbon becomes the Newman point and which the circle, transfer each substituent to its correct carbon first, then preserve the named dihedral angle rather than copying page positions mechanically.

Try this

Convert an anti-butane sawhorse viewed C2→C3 into a Newman projection.

Check your answer

The viewed-from carbon is in front: C2 is the point and C3 the circle. Keep one methyl on each carbon, staggered, with a 180° methyl–methyl dihedral.

Move between wedge–dash and sawhorse

A wedge–dash drawing can be rotated in space so the chosen C–C bond runs diagonally for a sawhorse view. Track towards/behind relationships while you rotate; the solid wedge is not permanently ‘up’.

For 1-bromo-2-chloroethane, label C1 and C2 before converting, then decide where each halogen sits and state the Br–C–C–Cl dihedral explicitly.

Try this

Represent a staggered anti conformer of 1-bromo-2-chloroethane as both a sawhorse and a Newman projection viewed C1→C2.

Check your answer

In both drawings Br belongs to front C1 and Cl to rear C2, and the anti Br–C–C–Cl dihedral is 180°.

Do not move substituents between carbons

Projection changes never alter which atoms are bonded.

A rear bond starts at the circle rim; it does not start at the central point and pass through the circle.

Try this

Better reasoning: ‘Both methyl groups may be placed on the front carbon to make anti-butane.’

Check your answer

Reject it because butane has one methyl substituent on each of C2 and C3.

Audit the representation

Check atom labels, connectivity, viewing direction, towards/behind information and named dihedral in that order.

Use wedge–dash, sawhorse and Newman forms confidently; Fischer projections are explicitly not required for this course.

In a drawing question, state the viewing direction, place every group on the correct atom, then name the spatial relationship shown. Those three checks make the diagram unambiguous even if it is rotated on the page.

Try this

State four checks for a valid conversion between stereochemical projections.

Check your answer

Name the atoms and sight line, preserve each atom's substituents, retain towards/behind information and verify the requested dihedral.