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CHEM 123L (14)


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Sue Stathopulos

Experiment 1: "A cis to trans conversion" Saturday, June 25, 2005 4:19 PM Notes on Theory Alkenes o They are hydrocarbons with a C=C double bond Double bonds are stronger and more reactive than single bonds o Hydrocarbons with double bonds used to be known as "olefins", because they had an oily appearance "Oleum" means oil "Ficare" means make Isomers o Geometric isomers have the same molecular formula, but a different geometric arrangement They also have different physical properties o Alkenes 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 o The 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 o o Heats of formation, denoted as f , are the change in heat for some system when a molecule is formed If they are negative, it means that heat is releasedmore 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 o This is when atoms or groups in a molecule are too close to each other, and so there is repulsion between their electron clouds o Consider 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 2Aso 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 apartmeaning that they are too close to each other! Hence the presence of steric hindrance o In 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 o sp3 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 o sp2 hybridization Here we have one 2s and two 2p orbitals combining to make 3 sp2 hybrid orbitals, leaving one p orbital unchanged 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 o sp1 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 waterthis will dissolve it into maleic acid o And 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 o This 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 o Note 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! o However, if water gets through the filter and the Buchner funnel, that's coolbecause 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 o So here is what refluxing is: You have a reaction going on in some vessel, but at
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