Figure 1 Delocalization of a positive charge
over the guanidinium group of Arginine

Figure 2 Arginine Amino Acid

Arginine        

                                                        

Arginine is one of 20 naturally occurring amino acids. It removes ammonia from the body and is important in both cell division and the healing of wounds. If taken as an oral supplement, Arginine can aid as a precursor to the synthesis of nitric oxide and urea, as well as lower blood pressure. It also plays a small role in the treatments of alcoholic hepatitis, advanced cirrhosis, and erectile dysfunction. It is found in the Lysine polypeptide, which binds free sugar molecules so they can react with proteins. This provides an anti-glycation effect.

Arginine is a white crystalline powder. Its molecular weight is 174g, it melts at 235˚C, and has an isoelectric point of 10.75. Its functional groups include four amines, a guanidinium group and one carboxylic acid. Amines absorb between 3300 and 3400 cm^-1 on the IR spectrum. The C=O bond of the carboxylic acid absorbs at about 1710 cm^-1 and the O-H absorption occurs from 2500-3500 cm^-1. The pK_a value for the carboxylic acid is approximately 5. The guanidinium group has a pK_a of 12.48. The nitrogen atoms of this group delocalize a positive charge. Figure 1 shows the guanidinium group. Figure 2 is larger picture of Arginine and is a more accurate depiction of the molecule, having a hydrogen atom bound to the oxygen in the carboxyl group.

http://en.wikipedia.org/wiki/Arginine

http://www.sciencelab.com/msds.php?msdsId=9927439

¹Smith, J. Organic Chemistry. 2^nd ed.; McGraw-Hill, 2008, pp 700, 974.

http://www2.chemistry.msu.edu/faculty/reusch/VirtTxtJml/proteins.htm

Electrophilic Aromatic Substitution Reaction in an Article

 In an attempt to find a peer reviewed article/ journal that discussed electrophilic aromatic substitution reactions, I accessed the JSTOR database, available through the school's website. Upon searching for the reaction, I found one titled: Electrophilic Mercuration and Thallation of Benzene and Substituted Benzenes in Trifluoroacetic Acid Solution. The article discussed the electrophilic aromatic substitution of a benzene ring with Hg(OCOCF₃)₂ to form C₆H₅Hg(OCOCF₃), which is then introduced with NaBr_(aq) to form C₆H₅HgBr. That compound was then introduced to Br₂ in CHCl₃ to form bromobenzene. The following is a link to the article, which includes the reaction mechanism.

 http://www.jstor.org/stable/67683?seq=3&Search=yes&searchText=aromatic&searchText=reactions&searchText=chemistry&searchText=substitution&searchText=electrophilic&list=hide&searchUri=%2Faction%2FdoBasicSearch%3Facc%3Don%26Query%3Dchemistry%2Belectrophilic%2Baromatic%2Bsubstitution%2Breactions%26gw%3Djtx%26acc%3Don%26prq%3Dchemistry%2Belectro-synthetic%2Baddition%2Breactions%26hp%3D25%26wc%3Don&prevSearch=&item=1&ttl=206&returnArticleService=showFullText&resultsServiceName=null 

In electrophilic aromatic substitution, a hydrogen atom on a benzene ring is replaced by an electrophile. In this mechanism, the carbon atom is attacked by the Hg atom in Hg(OCOCF₃)₂. The hydrogen atom on that carbon is then deprotonated by the NaBr _(aq).  The new substituent then undergoes halogenation to produce the bromobenzene product.