JAMES P. BIG Arizona State University Tempe, AZ 85287
f iltrote~ & refiduef Bromination of a Hydrocarbon Eva Deck Flat Rock High School, Flat Rock, MI 48134 Charles Deck BASF Corp.. Wyandotte. MI 48192 We have developed a simple one-hour student experiment that demonstrates photochemical halogen substitution in a quantitative way using heptane, bromine, and water. This reaction was discussed recently by Ivo Perinal as an optical projection classroom demonstration. Our experiment focuses on the quantitative nature of the reaction. As a control we place a bottle containing heptane, water, and bromine in a dark cunboard. T h e water added to the reaction mixture serves to dissolve the hydrogen bromide, which would otherwise build UD oressure in the bottle and be lost when the bottle is op&d. After 24 h the color of bromine in the controlsample is still there. The samples exposed to light are colorless within 10 min, even in the light of a cold winter day in Michigan. Caution: bromine is very corrosive and should be transferred in the fume hood and only hy the instructor. The experiment reinforces the knowledge of the actual nature o f t h e most common products of this reaction, which high school students frequently assume to be dibromoheptane and hydrogen. Experimental
buret clamp wash bottle centigram balance
Procedure Add 35 mL heptane and 15mL distilled water to the bottle. Make sure the hottle is dry on the outside. Place cap on the hottle. Find the mass of the hottle and liquids. The instructor will add approximately 1g of bromine to the hottle. Note: Due to the corrosive nature of the bromine it must he transferred in the fume hwd and onlv. hv. the instructor using proper safety terhniquea. Do not remove the cap from the bottle after the instructor transfers the bromine. Again find the mass of the bottle and contents. Calculate the mass oi the bromine. Shake the hottle. Place on a window sill. Shake the bottle every 2 min until the color disappears. Then shake once more. Meanwhile prepare the buret for titration. Wash the buret twice with 15 mL portions of 0.2 M NaOH. Fill the huret. Remove the cap from the bottle, and pour the contents into the Erlenmeyer flask. Rinse the bottle once with 20 mL distilled water. Add the rinse water to the Erlenmeyer flask. Add 2-3 drops phenolphthalein solution to the Erlenmeyer. Obtain the initial reading of the huret. Titrste to a pink endpoint. Read the huret again. ~~~~~
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Purpose To demonstrate the photochemical substitution reaction of a hydrocarbon by bromine. C,H,6
+ Br2
-
+
C7HlSBr HBr
Discussion Bromine is added to heptane and water. The reaCtion mixture is placed in bright light. The hydrogen bromide formed is extracted into the water when the bottle is shaken. The hromoheptane formed remains in the heptane layer. When the mixture is colorless, all the hromine has reacted. The aqueous phase containing HBr is titrated with 0.2 M NaOH. The moles of hydrogen bromide obtained should he equal to the moles of bromine added. Materials 2-02. bottle with cap 35 mL heotane 15 mL distilled water 1g bromine (handled only by instructor) 250-mL Erlenmeyer flask 100 mL 0.2 M NaOH phenolphthalein drops huret 'Perlna, I. J. Chem. Educ. 1986, 63.344.
Calculations Mass of bottle wlth bptane, water, and bromine Mass of bonk wlth heptane, water Mass of bromlns samde
B
9 9
mol mL mL mL
Initial reading buret Milliliter NaOH used In titration Malarity of NaOH Moles NaOH = Volume (liters)X Moiarity Moles NaOH = Moles HBr Moles HBr Agreement of moles bromine and moles HBr Sources of enor: Typical Student Results Mass of bromlne Moles bromlne Moles HBr Agreement Volume 66
1.03g 0.00645 mot 0.00658 mol 98 %
M
mol %
0.93 g 0.0058 mol 0.0059 mol 98%
Number 1 January 1989
75
