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Organic Lab Final
Terms in this set (74)
Would chloroform be a better or worse solvent for the extraction of of caffeine?
chloroform would be better because its distribution coefficient is higher than ethyl acetate
relationship between distribution coefficient and mass of caffeine left in aqueous layer
a higher distribution coefficient would result in lower mass of caffeine left in the aqueous later and higher mass transferred to the organic layer
trends in extraction (2x12 vs 3x8 vs 4x6
the higher the number of extractions, the more mass extracted
structure of benzoic acid
what part of benzoic acid makes it acidic?
the OH double bond O group
what part of phenacetin makes it neutral?
the double bond o and the o between the ethyl group and the benzene ring
chemical equation for the reaction of the organic salt of benzoic acid with hydrochloric acid, showing structures of the organic compounds
benzoic acid + NaHCO3 --> benzoic oxide + CO2 + H2O + Na+
maximum total volume that should be placed in separators funnels
2/3 of the rated capacity, 83.3mL for our funnels
when the spinach pigments were dissolved in acetone and then put into a separators funnel with diethyl ether, where would the acetone have been found in the funnel? Use property data to explain
Acetone is slightly more dense than diethyl ether and less dense than water. Therefore, it would be found between acetone and water if density where the only factor. However, we must consider that acetone is miscible in water. Therefore, the layers in the separators funnel and acetone dissolves into the lower, aqueous layer.
during the evaporation of the ether, the Erlenmeyer flask gets very cold. Explain why.
Ether is very volatile, so it evaporates quickly at room temperature. Because evaporation is endothermic, the action takes energy from the surroundings in the form of heat, therefore the surroundings become cold.
identify the stationary and mobile phases of the TLC from the think layer chromatography experiment
The silica gel coating on the TLC plate is the stationary phase. The eluent (70:30 hexane:acetone) is the mobile phase
silica gel structure
Explain how far polar vs. non-polar compounds should travel up the TLC plate based on the structure of silicon.
Silica gel is very polar and can hydrogen bond. More polar solutes have a higher affinity for the silica el, so they will not travel as high up on the TLC plate that less polar solutes. Therefore, MORE POLAR SOLUTES HAVE LOWER Rf VALUES
What is the origin on a TLC plate and how much (at what level) fluent should be placed in the TLC chamber relative to the origin?
The origin is the starting point of the TLC plate where the samples are initially applied, typically 1cm from the bottom of the TLC plate. The origin must be above the element.
Explain Rf values for the various components of the spinach extract based on the principles of thin-layer chromatography. Address stationary and mobile phases.
More polar solvents have an affinity for the polar stationary phase, less polar solvents have an affinity for the non polar mobile phase. Therefore, more polar solutes do not travel as high up on the TLC plates and have lower Rf values. Less polar solutes travel higher up the TLC plates and have higher Rf values.
What common method is used to visualize colorless compounds on a TLC plate?
fluorescent indicators and UV light
calculating Rf values equation
Rf= distance traveled / distance solvent front traveled
Which of the pigments identified is most polar and which is least polar. How do you know?
Xanthophyll with lowest Rf value are the most polar. Carotenes with highest Rf are least polar.
Put in order from least polar to most polar: acetone, water, cyclohexane, methanol, methylene chloride, and hexane
cyclohexane < hexane < methylene chloride, acetone, methanol, water
Identify the stationary and mobile phases of the column chromatography.
stationary phase: silica gel packing material
mobile phase: eluents (90:10 hexane:acetone and 70:30 hexane to acetone)
Explain why the yellow band should pass through the column before the green band.
The yellow band is composed of carotenes which are the most non-polar pigments. They will dissolve in the 90:10 hexane:acetone solution used first, but the green pigments will not. The green bands are made of more polar pigments, and they will need the 70:30 hexane:acetone solution, which is more polar, to travel through the silica packing material.
Explain what would have happened if just acetone was used to start eluting the column instead of the hexane-acetone mixture
Starting the column with the highly polar acetone would have made all the pigments dissolve, thus there would be no separation of pigments.
Explain what would happen if the 90:10 hexanes:acetone were added to the top of the column immediately after adding the sample without letting it adsorb onto the silica gel.
The various polarity pigments would not have adsorbed into the silica gel before adding the first fluent. If not given the chance to adsorb, all the pigments would immediately dissolve in the added solvent and elute together, therefore there would to no separation.
Which pigments were expected to be isolated using the 90:10 hexanes to acetone mixture followed by the 50:50 hexanes to acetone mixture?
Carotenes (less polar) would be extracted using 90:10 and the rest of the pigments (polar) would be extracted using 70:30
Which would you expect to react faster for Sn1 and Sn2 reactions: chlorine or bromine?
Bromine is expected to be a better leaving group than chlorine regardless of substrate or condition.
Would primary substrates be expected to react faster under Sn1 or Sn2 reactions?
1-bromobutane should have reacted faster than 1-chlorobutane under SN1 and SN2 conditions because Br- is a better leaving group than Cl-.
The rate of reaction should be faster in SN2 than SN1 because there is little steric hindrance by the primary substrate for the SN2 reaction mechanism and the carbocation intermediate of the primary substrate is least stable for the SN1 reaction mechanism.
Would secondary substrates be expected to react faster under Sn1 or Sn2 conditions?
2-bromobutane should have reacted faster than 2- chlorobutane under SN1 and SN2 conditions because Br- is a better leaving group than Cl-.
The rate of reaction could potentially be similar under SN1 and SN2 because of the moderate steric hindrance from the secondary substrate for the SN2 reaction mechanism and the moderate stability of the carbocation intermediate for the SN1 reaction mechanism.
Would tertiary substrates be expected to react faster under Sn1 or Sn2 reactions?
2-bromo-2-methylpropane should have reacted faster than 2-chloro-2-methylpropane under SN1 and SN2 conditions because Br- is a better leaving group than Cl-.
The rate of reaction should be faster under SN1 than SN2 because there is more steric hindrance by the tertiary substrate for the SN2 reaction mechanism and the carbocation intermediate of the tertiary substrate is the most stable for the SN1 reaction mechanism.
Explain the reactivity of crotyl bromide and benzyl bromide in comparison to other primary alkyl bromides.
Crotyl bromide and benzyl bromide should react faster than the other primary alkyl halides in SN1 because they have additional stability in their carbocation intermediate from resonance. Being primary alkyl halides they are expected to be as or more reactive than the other primary alkyl halides due to little steric hindrance in the SN2 reaction mechanism.
Explain the reactivity of bromobenzene in comparison to the reactivities of other secondary alkyl bromides
Bromobenzene is unreactive in both SN1 and SN2 conditions. The carbon-bromine bond is very strong and even though bromine is the better leaving group it will not leave the aromatic ring; plus phenyl carbocations are very unstable. Furthermore, the aromatic ring has high electron density making attack by nucleophiles unfavorable.
Give a brief summary of the Sn1 and Sn2 experiment
- 9 alkyl halides tested
-varied leaving groups (Cl and Br)
- varied substrates (primary, secondary, tertiary)
-Sn1 conditions were silver nitrate in ethanol
Sn2 solution was sodium iodide in acetone
-time it took for precipitate to form was recorded
Why is IR spectroscopy an important tool?
It is useful for identifying functional groups within a molecule.
Wat is the basis of IR spectroscopy?
Structural units within a molecule vibrate characteristically and the sensitivity to these group vibrations is the basis of IR spectroscopy.
What types of vibrational modes are shown for a Ch2 group?
CH2 groups stretch symmetrically and anti-symmetrically. They also bend by scissoring, rocking, wagging, and twisting.
What are the wavenumber and how are they expressed?
Wavenumber's are inverse wavelengths and the units are cm-1. They are on the x-axis
The region of infrared radiation falls between which two types of electromagnetic radiation?
The range of wavelengths for IR falls between UV and microwaves
The x-axis on IR spectra goes from high-energy to low-energy. what is the corresponding range of wavenmbers shown on IR spectra?
4000 cm-1 is the high-energy end of the scale and 625 cm-1 is the low-energy end. [The x-axis of an IR spectrum goes from 4000-500 cm-1.
Is the IR spectrum of a compound unique to that compound?
Two IR spectra of the same compound are the same, while any two different compounds are not the same. Therefore, every IR spectrum of a compound is unique.
Briefly describe methods used for taking IR spectra of solids and liquids.
Sodium chloride ["salt"] plates - A drop of liquid sample can be placed between two sodium chloride plates and then placed in the IR for determination. Solids can be dissolved in a solvent and also placed between the NaCl plates for determination.
Potassium bromide - Another method to determine IR of solids is to mix the solid with potassium bromide and press it into a wafer [or "pellet"] which is then placed in the IR for determination.
what is the fingerprint region and where is it found on IR?
The fingerprint region is the range of wavenumbers from 1500-500 cm-1 where the pattern of peaks varies most from compound to compound. [Peaks from 4000-1600 cm-1 are most useful for our purposes in identifying functional groups.]
Note: The fingerprint region is the "fingerprint" of a molecule in that if two spectra are superimposable including their fingerprint regions, they are identical compounds.
single bond stretching
tripple bond stretching
double bond stretching
single bond-stretching and bending
3500-3300, 2 peaks if primary, 1 weak if secondary
What does HNMR stand for?
Proton nuclear magnetic resonance spectroscopy
What does NMR spectroscopy depend on? What do HNMR spectrum tell us?
NMR depends on absorption of energy when the nucleus of an atom is excited from its lowest energy spin state to the next higher one. An 1H NMR spectrum tells us about the environments of teh various hydrogens in a molecule.
What are the essential features of an NMR spectrometer?
An NMR spectrometer consists of a powerful magnet, a radiofrequency transmitter, and a way to monitor the absorption of radiofrequency radiation and display the spectrum.
What does it mean to say that a proton is shielded?
rotons in a molecule don't feel the full strength of an external field because there are also local fields within the molecule from the electrons of various types of covalent bonds within the molecule - bonds with carbon, oxygen, nitrogen, etc. These local fields "shield" the protons from the external field so that the greater the shielding the greater must be the strength of the applied field to achieve resonance and produce a signal.
Are protons more shielded down field or upfield (lower frequencies)
Protons are LESS shielded downfield at higher frequencies (they are deshielded) and MORE shielded upfield at the lower frequencies.
Chemical shifts are measured along the x-axis in parts per million from a chemical standard used as the reference. What is the name, chemical formula, and abbreviation of this standard?
Tetramethylsilane (CH3)4Si, abbreviated TMS.
Which two solvents are used instead of chloroform and water to prepare solutions for NMR? Why is it necessary to replace those solvents with something else?
Chloroform and water are replaced by chloroform-d and deuterated water. The hydrogens in these solvents would show up on an HNMR, so solvents containing only deuterium (2H) must be used.
What is the effect of electronegative substituents on an organic compound and how is the effect seen looking at the chemical shift?
The shielding of protons increases as the electronegativity of the atoms vicinal to the proton decreases in electronegativity.
What three pieces of useful information are obtained from an NMR spectrum and what does each piece of information tell us?
1. The number of signals. This tells us how many different kinds of protons there are.
2. The intensity of the signals (as measured by the area under each peak). This tells us the relative ratios of the different kinds of protons.
3. The multiplicity (or splitting) of each signal. This tells us how many protons are vicinal to the one giving the signal.
What is multiplicity?
Multiplicity refers to the number of peaks the signal for a particular proton is split.
Multiplicity of signal for Ha = n + 1, where n is equal to the number of equivalent protons vicinal to Ha.
What does mass spectroscopy examine?
Mass spectroscopy examines ions produced from a molecule in the gas phase.
Write the equation that represents the collision of an electron at 10 eV with an organic molecule.
A:B + e- → [A.B]+ + 2e-
molecule + elecctron → cation radical + two electrons
What is the molecular ion?
The molecular ion is a cation radical that forms when the original molecule has been ionized by electron impact. The molecular ion has the same mass as the original molecule minus one electron. [A.B]+
What happens to the molecular ion as a result of electrons energized to 70eV?
At 70 eV electrons will not only ionize the molecule but will break chemical bonds. The molecular ion then breaks into smaller fragments.
[A.B]+ → A+ + B.
cation radical → cation + radical
What does m/z stand for? Are molecules with small m/z deflected more or less than those of larger m/z. Does the m/z increase or decrease moving from left to right on a mass spectrum.
m/z stands for mass-to-charge ratio. Ions of small m/z are deflected more than larger m/z; scanning all m/z values gives the distribution of positive ions for a particular compound. The x-axis of a mass spectrum goes from smaller to larger m/z from left to right.
What is the base peak on a mass spectrum? What is the intensity of a peak proportional to?
The base peak is the most intense peak on the spectrum and is assigned a relative intensity of 100. Ion abundances are proportional to peak intensities.
What is seen on a mass spectrum when an 1H is replaced with a 2H molecule? When a 12C is replaced by a 13C?
In both cases, there will be a peak one m/z unit higher than the molecular ion peak and every peak in the spectrum that contains hydrogen and carbon.
How can the presence of a single chlorine atom present in an organic molecule be detected on a mass spectrum?
Elemental chlorine is 35Cl and the most abundant isotope of chlorine is 37Cl. The natural ratio between the two is 100 : 32.7.
The ratio is approximately 3 : 1 (35Cl : 37Cl).
How can the presence of a single bromine atom present in an organic molecule be detected on a mass spectrum?
Elemental bromine is 79Br and the most abundant isotope is 81Br. They are two molar mass units apart and so the two peaks will also be two m/z units apart. The 100 : 97.5 ratio (79Br : 81Br) can be reduced to approximately 1 : 1.
Due to the isotope of chlorine, when two chlorine atoms are present in an organic molecule a pattern of peaks appear on a mass spectrum in a 9:6:1 ration. How many m/z units apart would these peaks be expected?
When two chlorine atoms are present, three peaks appear due to the three possible configurations. The second configuration, below, is two m/z units greater than the first configuration. Similarly, the third is two m/z units greater than the second. [The intensity of the three resulting peaks is approximately 9:6:1]
Due to the isotope of bromine, when two bromine atoms are present in an organic molecule a pattern of peaks appear on a mass spectrum in a 9:6:1 ration. How many m/z units apart would these peaks be expected?
Similar to having two chlorines in a molecule, two bromines would result in three peaks that are each two m/z units apart from each other due to the three possible configurations of having two Br atoms in a molecule.
largest peak in mass spec; all peaks are assigned abundances as percentages of the base peak
The nitrogen rule states that a molecule with an odd number of nitrogens has an odd molecular weight. A molecule with only C, H, and O or with an even number of nitrogens has an even molecular weight.
How can the index o hydrogen deficiency be used?
The index of hydrogen deficiency can be used to determine how many rings or double bonds are present in a molecule. For example, for an index of hydrogen deficiency of 1 there could be either one ring or one double bond present in the molecule.
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