D:

The first Ph Ch exam was hard. That was expected of course. What pains me though is that I hadn't been able to finish it. Anyway here's the reviewer I made for the said exam.

Sorry for the size. Just click on it to view it better. :)

Keep Calm and Carry On

Chem 18 exam on Wednesday and Chem18. 1 & Ph Ch 125 exam on Saturday. The dates are looming and I am not yet prepared. T___T

Resonance Structures

Lewis structures that are equivalent except for the placement of electrons are called resonance structures or resonance forms and the hypothetical switching from one resonance structure to another through electron delocalization is called resonance. By convention, resonance structures are separated by double headed arrows.

Seen above are the resonance structures of the ozone molecule. Notice that the only difference between the two structures is in the placement of the double bond and the placement of the lone pair.

Resonance structures are possible whenever there is a multiple bond and an adjacent atom with at least one lone pair. For example, the two resonance structures for the formate ion, HCO₂^- are

As represented by the arrows, in order to generate the second resonance structure from the first, one lone pair is turned into another bond, thus forming a double bond and consequently, this causes one bond to turn into a lone pair.

The general way of deriving resonance structures from one another is through shifting one of the lone pairs to and adjacent atom to form another bond and through shifting one of the bonds in a double or triple bond up to form a lone pair. This should not be applied to all cases though as the atoms' electronegativities should also be considered. For example, in fluoroethene, CH2CHF,

the resonance structure on the right is not reasonable as fluorine is the most electronegative element and in order to for it to form two bonds and two lone pairs, it would first have to lose an electron.

It is also possible to form resonance structures without the participation of lone pairs. An example is the benzene ring which involves a cycle of double bonds.

It should be taken into consideration that in drawing resonance structures, only the electrons may be moved. The nuclei of the atoms are fixed. The total charge and the total number of electrons are fixed as well.

With all the rules and all the other things to be considered, I find this lesson complicated. :c

3: Isomerism

Isomers are compounds with the same molecular formula but with different structural formulas. As part of the lesson, the professor drew structures in front and our task was to determine whether the said structures were isomers or just the same compound.

It is easy to determine that the compounds above are isomers based on which carbon the amino group (NH2) is attached. On the structure at the left, NH2 is attached to the 2nd carbon. For the one in the middle, NH2 is attached on the 3rd carbon and for the last one, NH2 is seen on the 4th carbon thus proving the difference on their structural formulas.

On this example, the compounds are also isomers as N is placed differently on each compound. To make sure that the compounds are really isomers and not just the same structure, one may try naming each one. Variations in the name generated such as with that seen above already indicate that their structural formulas differ from each other.

The concept of isomerism is basically simple to understand given that one already understands fully how to name structural formulas. In my perspective, the confusing part that may lead to errors in distinguishing identical structures from isomers is in the numbering of the longest carbon chain but once the skills in naming is mastered, I suppose that there wouldn't be further problems regarding the topic.