So ultraviolet light absorption every molecule can absorb ultraviolet light and what’s gonna happen is when a molecule absorbs ultraviolet light it can promote an electron from a lower energy state to a higher energy state ultraviolet light has a slightly shorter wavelength and thus higher frequency than visible light and it occurs mainly in the 200 to 400 nanometer
Range remember we see colors in the 400 to 700 50 nanometer range when electrons in a lower energy state known as the ground state absorb light having the appropriate energy an electron is promoted to a higher energy state known as the excited state the energy difference between the two states depends on the location of the electron you might remember this from
Chem 111 where we talked about h nu e equals h nu and the bohr model of the electron well what we’re gonna see is the energy difference between these two states depends on the location of the electron so if i know e equals h nu or e equals c over lambda i can then know how much energy is needed to promote that electron to the excited state with conjugated inés
Electrons in a sigma bond and unconjugated pi bonds requires light having a wavelength of less than 200 nanometers therefore we’re going to only see ultraviolet radiation absorbed in conjugated systems the conjugated ions are going to have a slightly longer wavelength and we would see with an isolated unconjugated pi system the wavelength of the uv light absorbed
By a compound is often referred to as its lambda max for example 1 3 butadiene has a lambda max of 217 nanometers whereas 1 3 cyclo hexa diene has a lambda max of 256 nanometers as you increase conjugation you increase delta max 1 3 butadiene has al and a max of 217 nanometers 1 3 5 hexatriene has a lambda max of 268 animators and when we finally go up to having
8 7 double bonds i can actually count today actually 6 sorry it has a lambda max of 364 nanometers compounds take on a color of the light it does not absorb in these cases let’s take a look at lycopene like a penis orbs visible light at 470 nanometers which is in the blue green region of the visible spectra because it does not absorb light in the red region like
A peanut peers to be bright red why is this important later on in the semester we’re going to perform an experiment where we’re going to synthesize a sunscreen ultraviolet radiation from the sun is high enough in energy to cleave bonds forming radicals that can prematurely age skin and cause cancer melanoma however since much of this radiation is filtered out
By the ozone layer only ultraviolet light having wavelength greater than 290 nanometers reaches the sir or surface much as this ultraviolet light is absorbed by melanin the highly conjugated colored pigment in the skin serves as the body’s natural protection against the harmful effects of ultraviolet radiation prolonged exposure to sun can allow more ultraviolet
Radiation to reach your skin than millington can absorb therefore people use commercial sunscreens to offer better protection against ultraviolet radiation because they contain conjugated compounds that absorb ultraviolet light thus shielding the skin for a limited time from exposure to harmful ultraviolet radiation commercial sunscreens are given an spf rating
Sun protection factor according to the amount of sunscreen present the higher the number the greater the protection to sunscreens have been used for this purpose our para amino benzoic acid pa ba and padme oh we were going to make a different sunscreen in lab at the end of the semester
Transcribed from video
Conjugated Dienes and UV light By VanNessChemVids