When atoms share electrons to bond, the first bond that forms between them is a sigma bond.
Sometimes, further bonding can occur between the same two atoms (which are already bonded together with a single sigma bond). If this can happen, they will le to form a double bond. If an extra two electrons are shared to form a pi bond. If they bond together even more after that, then a second pi bond will form between them.
A sigma bond is formed when the valance orbitals of one atom, directly overlap, head-on, with the valence orbitals of a different atom
When carbon, oxygen and nitrogen bond covalently to other atoms, they hybridize. Note: some atoms do not hybridize, but for this course we focus strongly on C,N,O which do hybridize.
You may notice the atoms, C,N and O have for their valence shell, the n=2 shell, involving the 2s orbital, the 2p(x) orbital, 2p(y) orbital and the 2p(z) orbital. Remember an orbital can only hold 2 e- and when this happens, they must have opposite spin!
Hybridization (mixing) of an atoms internal orbitals allows better overlap between atoms, hence stronger bonds form, hence more energy is released when they form, hence the resulting species is more stable than it would be if no hybridization takes place.
- If an s orbital hybridizes (mixes) with a p orbital on the same atom, you get an sp orbital.
- If an s orbital hybridizes (mixes) with two lots of p orbital on the same atom, you get an sp2 orbitals.
- If an s orbital hybridizes (mixes) with three lots of p orbital on the same atom, you get an sp3 orbitals.
The diagram below is to try and help illustrate that. It shows a 2p(x) orbital of one atom (e.g. a carbon atom) bonding/overlapping with an s orbital of another atom (e.g. a hydrogen atom).
Summary: Sigma bonds (single bonds) form by head on overlap of the atomic orbitals of different atoms
What about pi bonds?
Pi bonds are formed from NON-hybridized p orbitals overlapping in a sideways manner.
The electron density in the atoms is a pi bond is formed by the sideways overlap of non-hybridized orbitals. This is illustrated below (note the preceeding sigma bond between the C atoms is not shown for the sake of clarity, but in reality it is there)
A very useful statement:
The shortest distance between two points is a straight line. The e- in a sigma bond essentially lie in a straight line (with a bit of bulging) between the attracting nuclei. So the e- are at the shortest distance that’s possible between to two attracting nuclei, hence they are attracted (i.e. bonded) as strongly as they can be, and so are ‘tied’ up, and not so easily available for reaction.
The electrons in pi bond are NOT directly in between the attracting nuclei, so the distance of the attracting nuclei to the electrons in a pi bond is greater than it is for sigma bond.
HENCE, the e- in a pi bond are attracted less strongly than a corresponding sigma bond, so the e- in the pi bond are MORE reactive than the electrons in the sigma bond.
The result of this is that pi bonds are more reactive than sigma bonds, so pi bonds will react first. This helps you identify some reactive sites in molecules, and so you can begin to ‘feel’ how a chemical can react.
Skeletal diagrams show C-C bonds and bonds to heteroatoms (e.g. C,N,O,F,Cl,Br,I). Bonds between C and H are usually left off – especially if they are not involved in a discussion or a reaction.
Get familiar with using skeletal sketches of molecules. They will be used on the exam paper.
Dear students, please students to fill in the entrance survey in i-learn
portal before the third week of semester!
I will give you a hard copy (printed copy) of the labs and an instruction manual next week (Monday, 10th March). I will probably do a lab briefing IN the lab next week (10th-14th March). I’ll let you know in class again on Monday.
P.S. AS 229
Re: The timetable clash. I brought it to the attention of the timetable committee and they have registered each one of you for the lab. All you have to do is split yourselves into two EVENLY SIZED groups, one group (group “x”) doing organic chem on Tuesday morning, the other doing (‘y’) doing analytical. Then elsewhere in the week (thursday I think), ‘x’ will enter the analytical chem while group ‘y’ go for organic.
This was necessary because your group is quite large. Not enough lab spaces.
Hello there AS229-2A (46? students) and AS246-3A (14 students)
Welcome to CHM412 – Organic Chemistry For Technology
I hope you will use this blog to it’s full potential. Sadly, last semesters students hardly used it at all.
Clicking http://intechemistry.wordpress.com/2013/09/09/chm-412-sept-dec-2013/ will take you to the ‘home page’ of CHM412. Feel free to ask any question in the relevant, clickable, sub-pages listed there. I do ask that if you want to ask a question, then please state your current understanding of the issue you want to ask about.
This semester, because of past abuses, I will not give take-home tests, and may give a number of ‘spot tests’ instead. So you must attend every class or have a PROPER reason for not attending.
The code has not changed since last semester, hence the material here is just as relevant to you as it was for last semesters students.
Please read ALL that this blog has in relation to CHM412 and download all the necessary documentation.
Best wishes and I look forward to your succeeding in this code.
at 12:00pm (noon) today I’ll place your take home test papers AND a copy of the mark scheme in my mail-box outside my office. Because this test was a varient of past years papers (covering most of the syllabus) It will be given a LOT more weightage than the other two previous tests.
There is still a problem with test 1 and multiple choice test papers
Very very disappointed to see it on the take home test, despite declarations all the work was entirely the work of the individual.
Whether you like it or not, a part of exams is to demonstrate you can follow instructions. Failing to do so causes a penalty to the number of marks you can score in the test. One instruction given to you in exams is to write the questions that you did on the front cover of the examination booklet. I am tired of students being lazy that can’t be bothered to follow this simple instruction so expect an appropriate penalty.
CHM412 June 2013 Past Years Paper
Feel free to leave Q’s about this paper.
BEFORE ASKING: please mention Q number and part as well as your current understanding/answer.
Q1: Structures from given names: 10 marks
Q2: Names from given structures: 10 marks
Q3: “Aspects/Features” of molecules, hybridization, definitions and example plus isomer 10 marks
Q4: Alkanes, more features of molecules, bpt order, IMF’s 10 marks
At this stage we can note that half the total number of marks are avaialbe for very unchallenging questions. You need to maximise scoring on these Q’s in order to copw with any mark losses on the harder, more tricky questions.
Q5: Chemical reaction pathway 8 marks
Q6: carbonyl related interconversaions and tests and experiments done in the lab sessions plus sketching amines 15 marks
Q7: Chemical reactions (functional group conversions) 8 marks
Q8: Benzene related reactions 9 marks