Good for Nothing Alkanes

Let’s face facts: alkanes don’t have many uses.  In real life, they are non-polar solvents, larger-chaines ones can be used to make wax, and they are good for burning/they make good fuels.  As far as organic chemistry goes, alkanes are very boring.  They have only three uses in your class:

1)      Solvents: Alkanes are very good non-polar solvents.  See tip #XX for more information on this.

 

2)      Halogenation: Alkanes can be reacted with bromine or chlorine under free-radical conditions to obtain an alkyl halide.  See tip #XX for more information on this.

 

3)      Combustion: This is the fuel part.  Complete combustion is an alkane reaction with oxygen to obtain CO₂ and water, as shown in the following example:  C₃H₈ +5O₂ à 3CO₂ + 4H₂O + Heat

 

There is only a small possibility that you will see a combustion reaction on one of your exams.  Therefore, the only real uses for alkanes in your undergraduate organic chemistry class will be as a solvent or in a halogenation reaction.  Hence, if you have an alkane in one of your exam reactions, it should be very simple to determine its role.  If there is a halogen (usually either Br₂ or Cl₂) , then beware of a halogenation reaction.  Otherwise, it is most likely safe to say that if you have an alkane in your reaction, it is a solvent and does not participate as a reactant.

For more helpful organic chemistry information, please go to organic chemistry.

Helpful Chemistry Site of the Week.

Today’s site of the week is www.chemicalforums.com.  I have been a part of this site for a while, and have been pretty impressed with it so far.  Once you register, you can post chemistry questions for the experts to answer.  The experts are extremely knowledgable, and you get a bunch of responses in a very short time.  Great resource for the undergrad who wants a quick answer to a topic that has eluded them to this point.  An even better resource for the grad student who wants to run a research idea by a 10,000 lb brainiac.

This site gets 4.5 beakers out of 5.

For further information on this, please see organic chemistry.

Chemistry, Electricity and the Environment

During the election cycle, there was much discussion about our energy future.  I imagine that it clear to most that we can not continue to live on Middle Eastern oil forever, but what are the options?  Clean coal technology? Wind? Solar? Nuclear?  Natural Gas?  The choices are numerous and complex.  Further, and maybe more importantly, we must consider the impact on the environment. 

For those of you living in the great state of Texas, you have options.  One of them is Texas Electricity.  You have the choice to save on commercial electric, business electric and residential electric by switching to deregulated electric supply at Texas Electric Company. 

What impresses me is the ability to save the environment, with numerous “green” electric options.  Clean, renewable energy options will keep greenhouse gases like CO2 from being placed in our atmosphere and lead to a measurable decrease in global warming over the coming year.  You can do your part to help with that by visiting Texas Electricity. 

Pollution free electricity is clean, affordable and reliable power.  you can learn more about this and other options for your residential and business power needs by visiting Texas Electricity.  Remeber, this is our chance to leave the Earth better for our children than we found it.

Know your strong nucleophiles

This is important throughout organic chemistry, but will be especially important when trying to determine the products of elimination and substitution (E1, E2, SN1, SN2)reactions.  There are generally three trends to remember when discussing how nucleophilic a reactant is:

1)      Size - Generally, the more linear and/or smaller the nucleophile, the more nucleophilic it will be.  This is because it can react at more sites and will not be sterically hindered if it is smaller or linear.

2)       Electronegativity- The more electronegative an atom is, the less nucleophilic it will be.   This is because more electronegative atoms will hold electron density closer, and therefore will be less likely to let that electron density participate in a reaction.  We see this in calculations and experiments that show nucleophilicity decreases as you get closer to fluorine on the periodic table (C > N > O > F)

3)      Polarizability- The more polarizable an atom is, the more nucleophilic it will be.   Polarizability is defined as the ability to distort the electron cloud of an atom, which allows it interact with a reaction site more easily.  Generally, polarizability increases as you travel down a column of the periodic table (I > Br > Cl > F)

 Below is a table of relative nucleophilic strength.  This is relative because nucleophilic strength is also dependant on other factors in the reaction, such as solvent. 

VERY Good nucleophiles	HS-, I-, RS-
Good nucleophiles	Br-, HO-, RO-, CN-, N3-
Fair nucleophiles	NH3, Cl-, F-, RCO2-
Weak nucleophiles	H2O, ROH
VERY weak nucleophiles	RCO2H

 

As shown above, as a general rule, the anion of a reactant will be a better nucleophile than the neutral form.  (i.e. RCO₂^- is a better nucleophile than RCO₂H)

For more information on this and other topics of organic chemistry interest, please visit organic chemistry