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CHEM123L Study Guide - Maleic Anhydride, Steric Effects, Orbital Hybridisation


Department
Chemistry
Course Code
CHEM123L
Professor
Sue Stathopulos

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Experiment 1: "A cis to trans conversion"
Saturday, June 25, 2005
4:19 PM
Notes on Theory
Alkenes
oThey are hydrocarbons with a C=C double bond
Double bonds are stronger and more reactive than single bonds
oHydrocarbons with double bonds used to be known as "olefins", because they had an oily
appearance
"Oleum" means oil
"Ficare" means make
Isomers
oGeometric isomers have the same molecular formula, but a different geometric
arrangement
They also have different physical properties
oAlkenes exist as two different geometric isomers
This is because the double bond in an alkene cannot twist very much, so there
are two distinct geometric configurations
Cis and trans
oThe two isomers are generally named cis and trans
"Cis" means the same, which is because the cis isomer will have some important
molecular group on the same side of the molecule (think about what C4H8 looks
like)
"Trans" means opposite, so the important molecular groups will be on opposite
sides of the molecule
oHeats of formation, denoted as ΔHof, are the change in heat for some system when a
molecule is formed
If they are negative, it means that heat is released…more heat is released when
a reaction is more exothermic
Exothermic reactions happen naturally and thus their product is stable
Trans alkenes are more stable than cis alkenes by about 1 kcal/mole, which
means that their heats of formation are more negative
Steric hindrance
oThis is when atoms or groups in a molecule are too close to each other, and so there is
repulsion between their electron clouds
oConsider cis-2-butene, which has 2 CH3 groups on the same side…
If you measure the van der Waals radii (the radius of an imaginary sphere
surrounded the molecule) for one CH3 group, it is 2A…so if the two CH3 groups
were as close together as possible but not touching, their centers would be 4A
apart
However, they are measured to be about 3A apart…meaning that they are too
close to each other! Hence the presence of steric hindrance
oIn this lab, we are investigating the interaction between the cis and trans
isomers of 1,2-ethylenedicarboxylic acid
The cis isomer is also known as "maleic acid"
The trans isomers is also known as "fumaric acid"
Hybridization of carbon
osp3 hybridization
This involves one 2s and three 2p orbitals (do you know what an orbital is?)
forming 4 sp3 orbitals
The bonds between hybridized orbitals are called sigma bonds
The bonds between unhybridized orbitals are called pi bonds
osp2 hybridization
Here we have one 2s and two 2p orbitals combining to make 3 sp2 hybrid
orbitals, leaving one p orbital unchanged

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The p orbitals lie perpendicular to the plane of the hybrid orbitals
(remember that the p orbitals bond with a pi bond)
Remember that pi bonds are weaker than sigma bonds
osp1 hybridization
Only one 2s and one 2p orbital are hybridized here to form two sp hybrid orbitals
The p orbitals are (once again) parallel to each other, and perpendicular to the
line formed by the hybridized orbitals
Notes on Procedure
Put maleic anhydride into boiling water…this will dissolve it into maleic acid
oAnd it will be a supersaturated solution, which means that since the water is so hot
(boiling!), more maleic acid is dissolved in it than is normally possible
Then we let the solution cool, and also put it on ice
oThis lowers the temperature of the water enough that excess maleic acid (which now
cannot be dissolved in the water) crystallizes
Take the crystals out of the beaker by pouring the whole thing through a Buchner funnel (like a
coffee filter-type thing)…and use water to rinse the crystals out if they're sticking to the beaker
oNote that we shouldn't use too much water here, or else the crystals will just go right back
into solution since there is enough water to dissolve in!
oHowever, if water gets through the filter and the Buchner funnel, that's cool…because it'll
just add to the dissolved maleic acid which is already in water!
So now we have maleic acid in two forms: dissolved in water, and crystallized
Take the stuff that was dissolved in water (the filtrate) and REFLUX it…
oSo here is what refluxing is:
You have a reaction going on in some vessel, but at the top of the vessel we
have a condenser
So as the reaction happens and vapors are formed, they have nowhere to go but
up this condenser (it is like a pipe thing)
But there is cold water running through an outer wall of the condenser, so that as
the vapor travels up, it is condensed - and drips back down into the vessel at the
bottom
This allows the solution to happen without the loss of solvent!
oSo we add concentrated HCl and two boiling chips to this dissolved maleic acid which we
have, and we reflux for 10 minutes
When the reaction is done, crystallize the product once again by putting it in an ice bath
oAlso, suction filter it and wash with a little bit of cold water
oThe product is fumaric acid, by the way :P
Now we compare properties of our two isomers (maleic and fumaric acid)
oUse a MelTemp melting point apparatus to compare the melting points for each product
oAlso, place each of the crystals in water to compare the solubilities of each isomer
Notes on Results
The melting point of fumaric acid is higher than maleic acid
Fumaric acid is insoluble and maleic acid is highly soluble
Experiment 2: "The Structures of Crystals"
Sunday, June 26, 2005
3:40 PM
Notes on Theory
Solids normally exist as building blocks of atoms, molecules, or ions
oThese are assembled in an orderly manner into what is called a crystal structure
We describe these crystal structures (since there are different types) by their unit cell
oA unit cell is something which, when moved a distance equal to its own dimensions in
various directions, can generate the entire structure
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