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Shapes of Organic Compounds

Organic compounds are 3‐dimensional and their shapes depend on the preferred spatial arrangement of the covalent bonds. The shapes of organic compounds are best viewed with the aid of molecular models.
Tools Of The Trade 3 Dimensional Structures
© University of York

Organic compounds are 3‐dimensional and their shapes depend on the preferred spatial arrangement of the covalent bonds. The shapes of organic compounds are best viewed with the aid of molecular models.

Organic Compounds on Paper

To represent the 3‐dimensional shape on paper, we use hashed and wedged lines. A hashed line points behind the plane of the paper, the wedged line points in front, as shown here for methane (CH4).

tetrahedral

Methane has a tetrahedral shape, with carbon in the centre and the four hydrogen atoms pointing to the corners of a triangularly‐based pyramid. All the bond angles are 109.5°. (In ethane, H3C–CH3, the bond angles are near, although not exactly at, the tetrahedral value of 109.5°.)

Shapes of Larger Molecules

The structure gets more complicated for larger molecules. For butanal (CH3CH2CH2CHO), like for methane, we see that three carbon atoms have a tetrahedral shape, but the one in the aldehyde functional group (the purple carbon) is different. It has a trigonal planar, or flat, shape – with the aldehyde carbon in the centre, and oxygen, hydrogen and another carbon pointing to the corners of an equilateral triangle. The bond angles are 120°.

trigonalplanar

In summary:
4 groups of bonding electrons around the ‘middle’ carbon of the bond usually give bond angles of about 109° e.g. alkanes (4 groups = 109°)
3 groups of bonding electrons around the ‘middle’ carbon of the bond give bond angles of around 120° e.g. aldehydes and alkenes (3 groups = 120°)
2 groups of bonding electrons around the ‘middle’ carbon of the bond give bond angles of 180° e.g. alkynes (2 groups = 180°)

You can further explore the shapes of various organic compounds in the download below, called ‘molecular shapes’.

Hashed v Broken Lines

A hashed line, IIIII, indicates that the bond is extending behind the plane of the drawing surface. A broken line (- – – -) indicates that the bond is not a full bond. Instead, it is only a partial bond as in a hydrogen bond or a partially formed or broken bond in a transition state (although, sometimes hashed lines are also used for hydrogen bonds).

Typically, you will see hashed lines drawn using small lines changing to gradually larger width lines as you move away from the carbon (to the atom or substituent it is bonded to). However, some chemists dislike this and do the reverse; they argue that because the bond is pointing away from you, the larger lines should be closer to the carbon (and the smaller width lines should be closer to the atom or substituent), as the atom or substituent is further away from you. To avoid any confusion, many stick with equal width hashed lines (IIIIII) and find these are easier to draw (clearly) with pen and paper!

Chemistry Word Quiz

You will have noticed that organic chemistry has its own extensive vocabulary. For many learners this can be challenging, so as a recap of the vocabulary covered thus far, have a go at this quiz. See if you can solve these anagrams, using the clues provided (in brackets), and post your answers below.

  1. eat once (polish off)
  2. rumourers lists act (compare and contrast)
  3. a rich scan (keep someone sweet)
  4. ester talks culture (draw the line)
  5. element and pithy (two and two make five)
  6. aborted chart learn (get into shape)
  7. argon liar plant (flat out)
  8. dr bong hoyden (sign on the dotted line)
  9. pal counting four (chain gang)
  10. into tastier ants (step change)
© University of York
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