Solubility Properties of Organic Compounds Course Notes
Structural versus Stick Formulas
Structural formulas show the carbon and hydrogen atoms as well as the bond between them. Sometimes structural formulas show only the carbon atoms. The structural formula for 2-hexene is shown below .
C-C=C-C-C-C
Stick formulas are simpler. They represent the carbon skeleton of the organic molecule. Carbon and hydrogen are not utilized. Rather, carbon atoms are understood to exist at the ends of all "stick" segments and at the union of two "stick"segments. Hydrogen atoms are not drawn at all. The stick formula for 2-hexene is shown below.
Structural formulas show the carbon and hydrogen atoms as well as the bond between them. Sometimes structural formulas show only the carbon atoms. The structural formula for 2-hexene is shown below .
C-C=C-C-C-C
Stick formulas are simpler. They represent the carbon skeleton of the organic molecule. Carbon and hydrogen are not utilized. Rather, carbon atoms are understood to exist at the ends of all "stick" segments and at the union of two "stick"segments. Hydrogen atoms are not drawn at all. The stick formula for 2-hexene is shown below.
http://homepages.ius.edu/DSPURLOC/c122/images/sol01.gif
Hydrocarbons, Alkanes, Alkenes, and Alkynes
Hydrocarbons are organic molecules made up of only carbon and hydrogen. If the hydrocarbon has only single bonds between the carbon atoms , it is said to be an alkane. If there is a double bond between two carbon atoms, then the molecule is said to be an alkene. When there is a triple bond between two carbon atoms, that molecule is called an alkyne.
Benzene is a special case because there are three alternating double bonds within the six carbon ring, presenting chemical and physical properties that are not possessed by either alkanes or alkenes. Hydrocarbons with a benzene like ring within them are said to be aromatic hydrocarbons
Functional Groups
Functional groups are small groups of atoms within the molecule, whose location is where most of the compound's chemical reactions occur. These groups of atoms, the functional groups, almost always react the same way and have similar propeties no matter where on the carbon backbone they are stuck. Knowing that a functional group gives the organic compound its characteristic properties, if you can recognize the functional group it will be easier to understand and remember the organic chemistry reactions. For example - all alcohols, molecules with the -OH functional group will have the same characteristics and will react the same way. When you learn just one of these reactions, it applies to all members of the the alcohol family.
You should know the following functional groups:
1. alcohol http://homepages.ius.edu/DSPURLOC/c122/images/sol02.gif
2. ketone http://homepages.ius.edu/DSPURLOC/c122/images/sol05.gif
3. ether http://homepages.ius.edu/DSPURLOC/c122/images/sol03.gif
4. aldehyde http://homepages.ius.edu/DSPURLOC/c122/images/sol04a.gif
5. carboxylic acid http://homepages.ius.edu/DSPURLOC/c122/images/sol06.gif
6. ester http://homepages.ius.edu/DSPURLOC/c122/images/sol07a.gif
7. amine http://homepages.ius.edu/DSPURLOC/c122/images/sol08a.gif
8. amide, http://homepages.ius.edu/DSPURLOC/c122/images/sol09.gif
9. phenol http://homepages.ius.edu/DSPURLOC/c122/images/sol10.gif
10. alkenes
11. alkynes
Intermolecular Forces
There are two types of intermolecular forces we are concerned with in this lab: London Dispersion Force and the Hydrogen Bond.
London Dispersion
The forces that hold molecules together in a liquid, solid and solution phases are very weak. They are generally called London dispersion forces.
The electrons in the orbitals of a molecule are free to move around. If you could compare a "snapshot" of the molecule at an instant in time, you would see that there would be slightly different charge distributions caused by the different positions of the electrons in the orbitals. The amount of difference is based on the polarizability of the molecule, which is a measure of how well electrons can move around in their orbitals. In general, the polarizability increases as the size of the orbital increases; since the electrons are further out from the nucleus they are less strongly bound and can move about the molecule more easily.
When two molecules come together, these variations in charge can create a situation where one end of a molecule might be slightly negative and the other end of that molecule could be slightly positive. This would result in a slight attraction of the two molecules (until the charges moved around again) but is responsible for the attractive London dispersion forces all molecules have.
These London dispersion forces are weak, the weakest of all the intermolecular forces. Their strength increases with increasing size and polarizability of the molecule.
Hydrogen Bond
Hydrogen is a special element. Because it is really a proton, it turns out that it can form a special type of intermolecular interaction called the hydrogen bond. If hydrogen in a molecule is bonded to a highly electronegative atom in the second row only (N, O, or F), a hydrogen bond forms.
The three elements listed above will grab the electrons for itself, leaving the hydrogen atom with virtually no electron density (since it had only the one). Now, if another molecule comes along with a lone pair, the hydrogen will try to position itself near that lone pair in order to get some electron density back. This creates the hydrogen bond. The strength of this interaction, while not quite as strong as a covalent bond, is the strongest of all the intermolecular forces except the ionic bond.
Hydrocarbons, Alkanes, Alkenes, and Alkynes
Hydrocarbons are organic molecules made up of only carbon and hydrogen. If the hydrocarbon has only single bonds between the carbon atoms , it is said to be an alkane. If there is a double bond between two carbon atoms, then the molecule is said to be an alkene. When there is a triple bond between two carbon atoms, that molecule is called an alkyne.
Benzene is a special case because there are three alternating double bonds within the six carbon ring, presenting chemical and physical properties that are not possessed by either alkanes or alkenes. Hydrocarbons with a benzene like ring within them are said to be aromatic hydrocarbons
Functional Groups
Functional groups are small groups of atoms within the molecule, whose location is where most of the compound's chemical reactions occur. These groups of atoms, the functional groups, almost always react the same way and have similar propeties no matter where on the carbon backbone they are stuck. Knowing that a functional group gives the organic compound its characteristic properties, if you can recognize the functional group it will be easier to understand and remember the organic chemistry reactions. For example - all alcohols, molecules with the -OH functional group will have the same characteristics and will react the same way. When you learn just one of these reactions, it applies to all members of the the alcohol family.
You should know the following functional groups:
1. alcohol http://homepages.ius.edu/DSPURLOC/c122/images/sol02.gif
2. ketone http://homepages.ius.edu/DSPURLOC/c122/images/sol05.gif
3. ether http://homepages.ius.edu/DSPURLOC/c122/images/sol03.gif
4. aldehyde http://homepages.ius.edu/DSPURLOC/c122/images/sol04a.gif
5. carboxylic acid http://homepages.ius.edu/DSPURLOC/c122/images/sol06.gif
6. ester http://homepages.ius.edu/DSPURLOC/c122/images/sol07a.gif
7. amine http://homepages.ius.edu/DSPURLOC/c122/images/sol08a.gif
8. amide, http://homepages.ius.edu/DSPURLOC/c122/images/sol09.gif
9. phenol http://homepages.ius.edu/DSPURLOC/c122/images/sol10.gif
10. alkenes
11. alkynes
Intermolecular Forces
There are two types of intermolecular forces we are concerned with in this lab: London Dispersion Force and the Hydrogen Bond.
London Dispersion
The forces that hold molecules together in a liquid, solid and solution phases are very weak. They are generally called London dispersion forces.
The electrons in the orbitals of a molecule are free to move around. If you could compare a "snapshot" of the molecule at an instant in time, you would see that there would be slightly different charge distributions caused by the different positions of the electrons in the orbitals. The amount of difference is based on the polarizability of the molecule, which is a measure of how well electrons can move around in their orbitals. In general, the polarizability increases as the size of the orbital increases; since the electrons are further out from the nucleus they are less strongly bound and can move about the molecule more easily.
When two molecules come together, these variations in charge can create a situation where one end of a molecule might be slightly negative and the other end of that molecule could be slightly positive. This would result in a slight attraction of the two molecules (until the charges moved around again) but is responsible for the attractive London dispersion forces all molecules have.
These London dispersion forces are weak, the weakest of all the intermolecular forces. Their strength increases with increasing size and polarizability of the molecule.
Hydrogen Bond
Hydrogen is a special element. Because it is really a proton, it turns out that it can form a special type of intermolecular interaction called the hydrogen bond. If hydrogen in a molecule is bonded to a highly electronegative atom in the second row only (N, O, or F), a hydrogen bond forms.
The three elements listed above will grab the electrons for itself, leaving the hydrogen atom with virtually no electron density (since it had only the one). Now, if another molecule comes along with a lone pair, the hydrogen will try to position itself near that lone pair in order to get some electron density back. This creates the hydrogen bond. The strength of this interaction, while not quite as strong as a covalent bond, is the strongest of all the intermolecular forces except the ionic bond.
For hydrogen bonding to occur there are two criteria which must be present:
(1) a center of positive charge must exist on the hydrogen atom
(2) a center of negative charge must lie on an atom that has at least one lone pair of electrons (normally oxygen or nitrogen)
With these two criteria present, the hydrogen center of positive charge on one molecule and the negative center of charge on the other molecule attract each other establishing an intermolecular bond called a hydrogen bond.
These bonds are relatively strong because it involves the electrostatic attraction between permanent nonfluctuating centers of charge.
Functional groups that contain the O - H group are all water-like because they exhibit hydrogen bonding.
Trends in the forces
The intermolecular forces increase in strength according to the following:
London dispersion < dipole-dipole , H-bonding < ion-ion
Solubility
If the intermolecular forces between the molecules of one substance are roughly the same as the intermolecular forces between another substance, the two substances will probably dissolve in each other. This concept is condensed into these two rules:
Polar substances such as water might dissolve other polar substances such as alcohols and carboxylic acids. Water can be broken down into H - OH, thus it has the -OH group which identifies alcohol and carboxylic acids.
Nonpolar substances such as hydrocarbons dissolve other nonpolar substances, but do not tend to dissolve polar substances
Hydrophobic - Hydrophilic
When you are trying to evaluate the solubility properties of alcohols and carboxylic acids, it becomes necessary to consider the relative sizes of the hydrocarbon and water-like portions of the molecule. The hydrocarbon portion is said to be hydrophobic (water hating) because it will not hydrogen bond with water but does tend to dissolve in hydrocarbon liquids. The water-like alcohol and carboxylic acid groups hydrogen bond with water and are said to be hydrophilic (water-loving).
If the ratio of the size of the hydrophilic portion to the hydrophobic portion is small, the hydrophilic portion is too small to carry the molecule into solution with water. If the ratio is large, it can carry the molecule into solution.
The solubility of alcohols and carboxylic acids in water is made smaller when the hydrophobic portion of the molecule is made larger.
When determining the solubility of a molecule there is one final rule.
The solubility of hydrogen bonding molecules is improved if either the positive charge on the hydrogen is made larger or the negative charge on the electronegative atom (oxygen or nitrogen) is increased.
Extraction with a Separatory Funnel
Extraction methods take advantage of differences in the solubility properties of organic compounds. This procedure involves choosing an extraction solvent that is 1) immiscible with the solution solvent and 2) capable of solubilizing the desired organic to a greater degree than is the solution solvent.
We are using methylene chloride because our organic substances are either nonpolar or moderately polar and can be extracted from water.
The extraction solvent and the solution of caffeine-acetaminophen/NaOH are shaken together in the separatory funnel. Acetaminophen is insoluble in water but because of a weakly acidic phenol group it reacts with NaOH to produce soluble sodium acetaminophenoxide. Since caffeine is more soluble in methylene chloride than it is in water while sodium acetamonophenoxide is very soluble in water and virtually insoluble in methylene chloride this extraction will work very well.
Methylene chloride is more dense than water so methylene chloride-caffeine solution will be in the bottom layer which is drawn off. Acetaminophen stays in the top layer and will be drawn off last.
The extraction is performed three separate times with fresh solvent so we can extract the maximum amount of material possible. Three extractions of 15 mL of solvent will result in a significantly larger amount of material than one extraction which uses 45 mL of solvent.Separatory Funnel Technique
Boiling Chips
Reasons for using boiling chips:
1) To prevent superheating and bumping
2) To prevent violent formation of gas bubbles with severe splashing.
A liquid can be superheated or raised to temperatures above its boiling point, especially if it is heated rapidly. Bubble formation requires many high-energy molecules to gather in one place. If superheating occurs, the vapor pressure of the liquid is greater than the atmospheric pressure. Once the bubble forms, since its internal pressure is greater than the atmospheric pressure, it can burst before rising to the surface, blowing the surrounding liquid out of the container. This is bumping.
Boiling chips are porous ceramic material containing trapped air that escapes on heating, forming tiny bubbles that act as "starters" for vapor bubble formation. This allows a smooth onset of boiling.
