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Nomenclature
For the sake of clearer communication and less misunderstanding, a consistent naming system is necessary, which gives rise to the IUPAC nomenclature.
Some standard steps to follow when naming unsaturated hydrocarbon molecules with IUPAC nomenclature are elaborated below.
- 1. Find and count the number of carbon atoms in the longest carbon chain and use the corresponding number prefix. For example, if the longest carbon chain contains three carbon atoms, use prefix “prop-”. The prefix of number of carbons from 1 to 10 is summarized in the table below.
number of carbon atoms in the longest carbon chain |
prefix | number of carbon atoms in the longest carbon chain |
prefix |
---|---|---|---|
1 | meth- | 2 | eth- |
3 | prop- | 4 | but- |
5 | pent- | 6 | hex- |
7 | hept- | 8 | oct- |
9 | non- | 10 | dec- |
- 2. Determine the suffix based on the type of hydrocarbon.
- 3. Count the number of double bonds or triple bonds and indicate that by a number prefix before “-ene” or “-yne”. For example, a carbon chain with 4 carbon atoms containing 2 double bonds will be named as “butadiene”.
- 4. Add numbers between prefix of number of carbons and “-ene” or “-yne” to indicate the position of starting carbon of double bonds or triple bonds. For example, a carbon chain with 4 carbon atoms containing a double bond between the second carbon and the third carbon will be named as “but-2-ene”.
- 5. Lastly, use prefix before the prefix of number of carbons to indicate any side chains present. A straight carbon side chain is named simply by adding “-yl” after the prefix representing the number of carbon atoms in that chain. For example, if an ethyl group is attached to the second carbon in pent-2-ene, the molecule will be named as “2-ethylpent-2-ene”. For the naming of more complicated side chain, consult IUPAC nomenclature of organic chemistry. The side chain prefixes are added to the final name lexicographically, meaning an ethyl group will appear earlier than a methyl group.
- If the compound is circular, use prefix “cyclo-”. For example, a carbon ring with 5 carbon atoms containing 1 double bond will be named as “cyclopentene”.
Degree of Unsaturation
Degree of unsaturation is a calculation used to measure the number of
- DU = 2C+N-F-H+2/2
- C = number of carbon atoms in the compound
- N = number of nitrogen atoms in the compound
- F = number of halogen atoms in the compound
- H = number of hydrogen atoms in the compound
- the number of oxygen atoms or any other divalent atoms does not contribute to the degree of unsaturation
The degree of unsaturation also stands for that at most 2DU hydrogen atoms can be added to the compound to make it saturated.
Physical Properties
Boiling and Melting Point
This is a list showing the boiling points and melting points of saturated and unsaturated hydrocarbons with same number of carbons.[1][2]
Number of carbon | Melting/Boiling point(°C) | Alkane | Alkene | Alkyne |
---|---|---|---|---|
2 | Melting point | ethane -183 |
ethene -169 |
ethyne -80.7 |
Boiling point | ethane -89 |
ethene -104 |
ethyne -84.7 | |
3 | Melting point | propane -190 |
propene -185 |
propyne -102.7 |
Boiling point | propane -42 |
propene -47 |
propyne -23.2 | |
4 | Melting point | butane -138 |
1-butene -185.3 |
1-butyne -125.7 |
Boiling point | butane -0.5 |
1-butene -6.2 |
1-butyne 8.0 | |
5 | Melting point | pentane -130 |
1-pentene -165.2 |
1-pentyne -90.0 |
Boiling point | pentane 36 |
1-pentene 29.9 |
1-pentyne 40.1 |
Just like their saturated counterparts, the unsaturated hydrocarbons are usually non-polar. This means the intermolecular forces between unsaturated hydrocarbon molecules are dominantly weak Van der Waals force. The boiling point and melting point of unsaturated hydrocarbons are usually similar as their saturated counterparts with same number of carbon.
The melting and boiling points of unsaturated hydrocarbons compared to saturated ones are determined by two opposing factors. On the one hand, the strength of Van der Waals force depends on the number of electrons in a molecule. Unsaturated hydrocarbons have less electrons than saturated ones, so the boiling and melting point may decrease as intermolecular force decreases. On the other hand, the delocalized
The boiling and melting points also depend on the stereochemistry. The cis alkenes, due to their U-bending shape, cannot arrange themselves as closely as the trans ones, so they will have lower boiling and melting points.[1]
For longer chains of unsaturated hydrocarbons, the effects above still apply. In longer chains, the stereochemical "zig-zag" effect of unsaturated hydrocarbons become the dominant effect, so unsaturated long chain hydrocarbons usually have lower boiling and melting points.[3] The melting point difference between saturated and unsaturated fat inside human body also leads to health issues.
Solubility
Structure
Spectroscopic Properties
Compared to saturated hydrocarbons, the unsaturated hydrocarbons not only contains the C−C bonds and C−H bonds, but also have C=C double bonds and C≡C triple bonds. As a result, the spectrum will also contain characteristics of these
Infrared Spectroscopy
The stretching of C=C bond will give an IR absorption peak at 1670–1600 cm−1, while the bending of C=C bond absorbs between 1000–650 cm−1 wavelength. The stretching of C≡C bond absorbs 2100–2140 cm−1(monosubstituted) and 2190–2260 cm−1(disubstituted).[4] The strength of these absorption peaks varies with the place and number of the double or triple bonds.
Because of the delocalized
At the mean time, the absorption peaks of C–H and C–C bond, which are shared with the saturated hydrocarbons, also shows in the IR spectrum of unsaturated hydrocarbons.
NMR Spectroscopy
In 1H NMR spectroscopy, the hydrogen bonded to the carbon adjacent to double bonds will give a
In 13C NMR spectroscopy, compared to the saturated hydrocarbons, the double and triple bonds also deshiled the carbons, making them have low field shift. C=C double bonds usually have chemical shift of about 100–170 ppm.[7]
Chemical Properties
Combustion
Like most other hydrocarbons, unsaturated hydrocarbons can go under combustion reactions that produces carbon dioxide and water in complete combustion. The reaction equation is:
- CxHy + y+2x/2O2 → yH2O + xCO2
In the absence of oxygen, the combustion will turn into incomplete combustion and produce carbon monoxide and carbon.
The unsaturated hydrocarbons will produce incomplete combustion product more easily than saturated ones. As a result, the combustion of unsaturated hydrocarbons usually have yellow flame, different from the blue flame of the saturated ones. This indicates unsaturated hydrocarbon combustion will involve multi-step mechanisms, and the burning of carbon gives the yellow flame color.
Since unsaturated hydrocarbons have less hydrogen content, it will produce less water and decrease the flame moisture, as well as decrease the oxygen use. Acetylene(ethyne), for example, can be uesd as fuel.[8]
Compared to the single
Number of Carbon | Substance | Type | Formula | Hcø(kJ/mol) |
---|---|---|---|---|
2 | ethane | saturated | C2H6 | −1559.7 |
ethene | unsaturated | C2H4 | −1410.8 | |
ethyne | unsaturated | C2H2 | −1300.8 | |
3 | propane | saturated | CH3CH2CH3 | −2219.2 |
propene | unsaturated | CH3CH=CH2 | −2058.1 | |
propyne | unsaturated | CH3C≡CH | −1938.7 | |
4 | butane | saturated | CH3CH2CH2CH3 | −2876.5 |
but-1-ene | unsaturated | CH2=CH−CH2CH3 | −2716.8 | |
but-1-yne | unsaturated | CH≡C-CH2CH3 | −2596.6 |
Electrophilic Addition
The double or triple bonds that must present in unsaturated hydrocarbons provide high electron density that make the molecules become perfect spots for electrophilic addition reactions. In this kind of reaction, one
Hydrogenation
Hydrogenation is the electrophilic addition of hydrogen gas to unsaturated hydrocarbon. The result will be a more saturated hydrocarbon, but not necessarily become a saturated one. For instance, semihydrogenation of an alkyne may form an alkene. Nonetheless, the total number of
The reaction equation of hydrogenation of ethene to form ethane is:
- H2C=CH2 + H2→H3C−CH3
The hydrogenation reaction usually requires catalysts to increase its rate.
The total number of hydrogen that can be added to an unsaturated hydrocarbon depends on its degree of unsaturation. An unsaturated hydrocarbon with formula of CXHY can have 2X+2−Y hydrogen atoms at most added to it. This will make the molecule become saturated.
Halogenation
Similar as hydrogen, the heterolysis of halogen(X2) will produce a electrophilic X+ ion, after which it will be attacked by the electron on the
The reaction equation for bromine addition of ethene, for example, is:
- H2C=CH2 + Br2→H2CBr−CH2Br (trans)
Bromine test is used to test the saturation of hydrocarbons.[10] The test involves the addition of bromine water to the unknown hydrocarbon; If the bromine water is decolourized by the hydrocarbon, which is due to halogenation reaction, it can then be concluded that the hydrocarbon is unsaturated. If it is not decolourized, then it is saturated.
The bromine test can also determine the degree of unsaturation for unsaturated hydrocarbons. Bromine number is defined as gram of bromine able to react with 100g of product.[11] Similar as hydrogenation, the halogenation of bromine is also depend on the number of
Hydration
The
The reaction equation for hydration of ethene is:
- H2C=CH2 + H2O→H3C-CH2OH
The
The reaction equation of hydration of ethyne to form acetaldehyde is:
- HC≡CH + H2O → H2C=CH−OH
- H2C=CH−OH ⇌ H3C−CHO
Hydrohalogenation
The hydrohalogenation involves addition of H−X to unsaturated hydrocarbons. This will decrease one
The reaction equation of HBr addition to ethene is:
- H2C=CH2 + HBr→H3C−CH2Br
Polymerization
Unsaturated hydrocarbons have C=C double bond, making them perfect monomers for addition polymers.
Oxidation
Oxidation of unsaturated hydrocarbons depends on the strength of oxidizing agent. A weak oxidizing agent will lead to dihydroxylation, removal of one
A stronger oxidizing agent, for example KMnO4 or ozone, will lead to oxidative cleavage. In this case, the
Allylic Substitution
The
An example of this is NBS bromination reaction with alkene. The N−Br bond in NBS is weak so that much Br free radical will form. The free radical will attack the weakened allylic hydrogens and substitute them with bromine atoms.The reaction equation is:
- RCH2CH=CH2 + (CH2CO)2NBr → RCHBrCH=CH2 + RCH=CHCH2Br + (CH2CO)2N[20]
The reaction will produce two isomers with bromine attatched to different carbons. The reaction requires high amount of Br free radicals instead of electrophilic Br+ ions, which will go under addition reaction. NBS is essential to make such condition.[21]
If hydrocarbon groups are attatched to allylic carbon, it will make this carbon be more saturated. According to Zaitsev's Rule, this carbon will form a more stable carbocation intermediate. As a result, allylic rearrangement will occur, and the
Cycloaddition
For unsaturated hydrocarbons, ring structure and
Alkynes, under metal catalysts, for example cobalt, can also go under cycloaddition reaction called alkyne trimerization. Three alkynes goes under a "2+2+2" cyclization reaction and rapidly join together to form a benzene.Trimerization of different alkenes are usually not selective, but specially designed catalysts may increase the selectivity.[24]
Ligand Addition
The delocalized
The DCD model can also describe the alkyne ligand structure. Metal complex can also be intermediate of trimerization of alkynes, so metals can be catalysts of the reaction.
The synthesis of alkene ligand complexes can be described as a electrophilic addition reaction.
Similar as linear unsaturated hydrocarbons, the arene also have delocalized
Health Effects
Generally speaking, unsaturated hydrocarbon is healthier for body comparing to saturated hydrocarbon. This is because saturated hydrocarbons overall appear to be in straight chains, which are easier to stack together and form cluster. This effect is more significant in fats (lipids) (Lipids are not hydrocarbons since they contain oxygen). Therefore, as vegetable fats are generally unsaturated fats while animal fats are generally saturated fats, people may want to consume more vegetables fats for a healthier diet.
References
- ^ a b c Nguyen, Trung; Clark, Jim (April 23, 2019). "Physical Properties of Alkenes". Chemistry LibreTexts. Retrieved May 27, 2019.
- ^ Ophardt, Charles (2003). "BOILING POINTS AND STRUCTURES OF HYDROCARBONS". Virtual Chembook. Retrieved May 27, 2019.
- ^ Ophardt, Charles (2003). "Fatty Acids". Virtual Chembook. Retrieved May 29, 2019.
- ^ "IR Spectrum Table & Chart". Sigma-Aldrich. Retrieved May 5, 2019.
- ^ Merlic, Craig A. "Table of IR Absorptions". Webspectra. Retrieved May 5, 2019.
- ^ Hanson, John. "Overview of Chemical Shifts in H-NMR". ups.edu. Retrieved May 5, 2019.
- ^ a b "Nuclear Magnetic Resonance (NMR) of Alkenes". Chemistry LibreTexts. April 23, 2019. Retrieved May 5, 2019.
- ^ "Acetylene The hottest and most efficient fuel gas". Linde. Retrieved May 5, 2019.
- ^ "Organic Compounds: Physical and Thermochemical Data". ucdsb.on.ca. Retrieved May 5, 2019.
- ^ R.L. Shriner, C.K.F. Hermann, T.C. Morrill, D.Y. Curtin, and R.C. Fuson (1997). The Systematic Identification of Organic Compounds. John Wiley & Sons. ISBN 0-471-59748-1.
{{cite book}}
: CS1 maint: multiple names: authors list (link) - ^ "Bromine Number". Hach company. Retrieved May 5, 2019.
- ^ Clark, Jim (November 2007). "The Mechanism for the Acid Catalysed Hydration of Ethene". Chemguide. Retrieved May 6, 2019.
- ^ "Hydration of Alkynes". Chem LibreTexts. May 2, 2019. Retrieved May 6, 2019.
- ^ "Reactions of Alkynes - Addition of HX and X2". Chem LibreTexts. May 2, 2019. Retrieved May 6, 2019.
- ^ Kennepohl, Dietmar; Farmer, Steven (February 13, 2019). "Oxidation of Alkenes - Epoxidation". Chemistry LibreTexts. Retrieved May 27, 2019.
- ^ Kennepohl, Dietmar; Farmer, Steven (February 13, 2019). "Oxidation of Alkynes". Chemistry LibreTexts. Retrieved May 27, 2019.
- ^ Kennepohl, Dietmar; Farmer, Steven (May 22, 2019). "Oxidation of Alkenes - Cleavage to Carbonyl Compounds". Chemistry LibreTexts. Retrieved May 27, 2019.
- ^ Kennepohl, Dietmar; Farmer, Steven (May 10, 2019). "Oxidative Cleavage of Alkynes". Chemistry LibreTexts. Retrieved May 27, 2019.
- ^ "Radical Allylic Halogenation". Chem LibreTexts. June 30, 2018. Retrieved May 6, 2019.
- ^ Reusch, William (October 19, 2013). "Allylic Substitution". Chem LibreTexts. Retrieved May 6, 2019.
- ^ Ashenhurst, James (November 25, 2013), "Allylic Bromination", Master Organic Chemistry, retrieved May 6, 2019
- ^ Ashenhurst, James (December 2, 2013), "Bonus Topic: Allylic Rearrangements", Master Organic Chemistry, retrieved May 6, 2019
- ^ "Diels-Alder Reaction". Organic Chemistry Portal. Retrieved May 27, 2019.
- ^ Galan, Brandon; Rovis, Tomislav (July 7, 2010). "Beyond Reppe: Building Substituted Benzenes via [2+2+2] Cycloadditions of Alkynes". PubMed Central®. Retrieved May 27, 2019.
- ^ Toreki, Rob (March 31, 2015). "Alkene Complexes". Organometallic HyperTextbook. Retrieved May 29, 2019.
- ^ Evans, Michael (October 15, 2018). "
π Systems". Chemistry LibreTexts. Retrieved May 29, 2019.