Lab Report: Reactions of Hydrocarbons (Exp. 3) 40 pts/max
NAME: Shawn Vuong
SECTION: 145
Unknown Number: #4
1. Identity of Unknown (cyclohexane, cyclohexene, toluene or bromobenzene):
(5 pts)
Toluene
Experimental Evidence:
Chemical Tests:
The unknown reacted in bromine immediately after being exposed to light, turned solution from
brown to clear. Unknown compound did not react to Potassium Permanganate. Unknown compound
did react to Sulfuric acid, turned the solution light yellow.
(5 pts)
Infrared Spectrum: Identify all characteristic absorption bands of your unknown (5 pts).
The absorption bands present on my infrared spectrum for C-H bonds are as follows: according to
lab TA and Appendix 5 in Chemistry Lab Manual, -C-H (alkane) 3000-2800, =C-H (alkene) 31003000, and =C-H (aromatic) 3150-3050
2. Open the UCAN program. Which intermediate is formed at the first step of chain propagation for
the reaction of bromination of propane? (3 pts).
CH3-CH(+an Election)-CH3 is the free radical intermediate
Open the ORA program. Which step of bromination is shown on the animation? (2 pts).
It’s showing the chain propagation step. In which hydrogen is breaking a double bond, and a bromine
ion attaches to a central carbon.
13
C NMR Spectrum: Identify all resonances of your unknown (5 pts).
The resonances for NMR spectrum for the Unknown were, 21.790ppm which probably represented
the methyl group on the toluene. 125.646ppm, 128.578ppm, 129.380ppm, and 138.134ppm, which
represented each carbon/carbon couple as they proceeded away from the methyl group.
What evidence from this experiment supports the fact that the reactions of alkanes and alkenes with
Br2 occur by different mechanisms? (5 pts).
During the experiment the alkane cyclohexane reacted to Br2 but only with the addition of light. The
alkene cyclohexene did not need the light to fully react with the Br2. This shows that each reaction
was occurring because of a different existing mechanism.
3. The benzene ring of aromatic hydrocarbons contains three C=C double bonds. From your
observations in this experiment, do the C=C double bonds of the benzene ring show similar reactivity
to C=C double bonds of alkenes? Support your answer with experimental evidence. (5 pts).
No, the alkene, in this case cyclohexene, and the benzene rings (toluene & bromobenzne) reacted
very different in the experiment. The alkene (cyclohexene) reacted immediately to the Br2 while the
toluene reacted to Br2 with the aid of light and bromobenzene did not react to the Br2 at all. Also the
alkene (cyclohexene) reacted to potassium Permanganate (KMnO4) while both bromobenzene and
toluene did not react or hardly react at all. These show very different reactivity between alkenes and
benzenes.
4. In lab you will receive a list of three compounds and their IR spectra.
a. Match each compound to the correct spectrum.
b. Assign all of the characteristic absorption bands in each spectrum. For the IR-spectrum,
consider only the area 4000-1200 cm-1 (5 pts).