Alkenes

Alkenes are hydrocarbons that contain a double bond between two carbons, and are sometimes referred to by their old name, “olefin”. You might see this in some older, American textbooks.

Double bonded carbons show sp2 hybridisation, rather than sp3, (alkanes). In sp2 atoms, the s orbital has hybridised with 2 of the 3 available p orbitals, forming a trigonal planar arrangement of 3 sp2 orbitals.

This arrangement leaves a p orbital unhybridised, which sits perpendicular to the plane of the sp2 orbitals. This p orbital forms the pi bond of the double bond.

Naming alkenes:

The nomenclature of alkenes is much the same as alkanes:

  1. Find the longest carbon chain, which contains the double bond and name it as though it were an alkane, but change the end from –ane to –ene.
  2. Number the chain so as to include both carbon atoms of the double bond, and begin numbering at the end of the chain nearer the double bond. Use the number of the first carbon atom in the double bond to designate its position.
  3. Number the other substituents
  4. If you have a cycloalkene, number the carbons so the double bond has positions 1 and 2.
  5. Designate the geometry of the double bond with cis/trans or E/Z.

Name the following alkenes:

Questions taken from "Solomans Fundamentals of Organic Chemistry" 1994

E/Z isomerism:

Cis and trans can be used to designate the geometry of an alkene, provided it is disubstitued.

As soon as the alkene is tri or tetra substituted, cis and trans become meaningless, so E/Z is more correctly used for alkenes. (note: cis/trans is used for cycloalkanes).

E and Z designation follow the Cahn-Ingold-Prelog rules for determining priority of a substituent. The priority of each group on one carbon atom of the double bond is determined, then that is repeated for the second carbon atom. If the higher priority on both carbon atoms are on the same side of the double bond, (see cis), then it is Z (zusammen, German for together). If they are on opposite sides, (see: trans), it is E (entgegen, German for opposite).

Name the following alkenes, including E/Z and R/S geometries where required:

Questions taken from Solomans "Fundamentals of Organic Chemistry" 1994

http://www.chemguide.co.uk/basicorg/isomerism/geometric.html

Advertisements

Leave a comment

Filed under Chem 1, Organic Chemistry

Unit conversion

I often get asked about unit conversion, and it is a common mistake made in exams and prac classes. Units are very important, your answer without a unit means absolutely nothing! So, it’s important to get it right.

In the metric system, you need to know some of the prefixes associated with orders of magnitude.

tera- (T-) 1012
giga- (G-) 109
mega- (M-) 106
kilo- (k-) 103
hecto- (h-) 102
deka or deca- (da-) 10
deci- (d-) 10-1
centi- (c-) 10-2
milli- (m-) 10-3
micro- (µ-) 10-6
nano- (n-) 10-9
pico- (p-) 10-12

You need to multiply by a factor of 1000 to go between each prefix shown above.

There are a series of tutorials below, many of which have worked examples and problems for you to try:

http://chemistry.about.com/od/convertcalculate/a/conversions.htm

http://www.tutor-homework.com/Chemistry_Help/Unit_Conversion.html

http://www.learnchem.net/tutorials/mathb.shtml

http://pages.towson.edu/ladon/unit.html

Leave a comment

Filed under Chem 1

Isomerism in organic molecules

Isomers are compounds, which have the same chemical formula, but a different structure. There are several classes of isomer.

Structural isomers (also called constitutional isomers):

Structural isomers are molecules that have the same formula, but are connected in different ways. This can include branching of a chain, different positions of functional groups or the splitting up of a functional group, (for example, a carboxylic acid may become a ketone and an alcohol).

Conformational isomers (also rotational isomers or “rotamers”):

Conformational isomers occur when there is rotation around a single bond. It is less important for small, chain molecules as they are constantly rotating, but it is very important for bulky or cyclic alkanes.

Stereoisomers, (also geometric or optical isomers):

This broad class of isomers includes any molecule with the same order of atoms, but different geometries, or different arrangement of the atoms in space. There are two different types of stereoisomers:

Enantiomers: non-superimposable mirror images.

Non-superimposable mirror images

Diastereomers: stereoisomers that are not mirror images. These can include cis-trans (or E-Z) isomers, and molecules with more than one stereocentre.

To get stereoisomerism, you need a chiral molecule – one that is not identical to its mirror image. The most common example of chirality is your hands – your left and right hands are mirror images of each other, but you can’t superimpose them.

Chiral molecules can be spotted by looking for a chiral (or stereo) centre. This will be a carbon with four different substituents attached to it.

It is important to be able to distinguish between different enantiomers, so we use the R/S nomenclature. To designate R or S, you use the Cahn-Ingold-Prelog rules:

1. Prioritise the atoms directly attached to the chiral centre by atomic number.

2. If two or more atoms are the same, look at the next atom out, continuing until you find a point of difference.

3. Multiple bonds are counted as the same number of bonds to the same atom, (for example, a double bond to an oxygen counts as two single bonds to oxygen atoms)

Now, with the 4th priority substituent pointing into the page, draw a curved line from priority 1 to 2 to 3. If it is clockwise, the molecule is R, anticlockwise is S.

This molecule is S

Here are some links that might be useful:

http://www2.chemistry.msu.edu/faculty/reusch/VirtTxtJml/sterism3.htm

http://facultystaff.vwc.edu/~jeaster/courseinfo/Tutorials/stereochemistryl.html

2 Comments

Filed under Chem 1, Organic Chemistry

Youtube Friday

A Christmas special!

 

 

Leave a comment

Filed under Youtube Friday

Youtube Friday

Leave a comment

Filed under Youtube Friday

Youtube Friday

I know… I know, it isn’t Halloween. I just couldn’t help myself. 

Leave a comment

Filed under Uncategorized

Youtube Friday

This is a pet-hate of mine too.

Leave a comment

Filed under Youtube Friday