Chapter 11: Alkanes
What are the first ten alkanes with unbranched chains?
Methane (CH4) Ethane (C2H6) Propane (C3H8) Butane (C4H10) Pentane (C5H12) Hexane (C6H14) Heptane (C7H16) Octane (C8H18) Nonane (C9H20) Decane (C10H22)
What are the rules of the IUPAC system for naming alkanes?
1. The name for an alkane with an unbranched chain of carbon atoms consists of a prefix showing the number of carbon atoms in the parent chain and the suffix -ane. 2. For branched-chain alkanes, take the longest chain of carbon atoms as the parent chain and its name becomes the root name. 3. Give each substituent on the parent chain a name and a number. The number shows the carbon atom of the parent chain to which the substituent is bonded. Use a hyphen to connect the number to the name. 4. If there is one substituent, number the parent chain from the end that gives the substituent the lower number. 5. If the same substituent occurs more than once, number the parent chain from the end that gives the lower number to the substituent encountered first. Indicate the number of times the substituent occurs by a prefix di-, tri-, tetra-, penta-, hexa-, and so on. Use a comma to separate position numbers (2,4-Dimethylhexane) 6. If there are two or more different substituents, list them in alphabetical order and number the chain from the end that gives the lower number to the substituent encountered first. If there are different substituents in equivalent positions on opposite ends of the parent chain, give the substituent of lower alphabetical order the lower number (3-Ethyl-5-methylheptane not 3-methyl-5-ethylheptane) 7. Do not include the prefixes di-, tri-, tetra-, and so, on or the hyphenated prefixes sec- and tert- in alphabetizing. Alphabetize the names of substituents first and then insert these prefixes (4-Ethyl-2,2-dimethylhexane, not 2,2-Dimethyl-4-ethylhexane)
What are the prefixes used in the IUPAC system to show the presence of 1 to 20 carbons in an unbranched chain?
16 is hexadecane 17 is heptadecane 18 is octadecane 19 is nonadecane
What is a HYDROCARBON?
A compound composed of only carbon and hydrogen.
What is a cyclic hydrocarbon?
A hydrocarbon that contains carbon atoms joined to form a ring
What is an unsaturated hydrocarbon?
A hydrocarbon that contains one or more carbon-carbon double bond, triple bond, or benzene ring is classified as an unsaturated hydrocarbon.
What is an axial position?
A position on a chair conformation of a cyclohexane ring that extends from the ring parallel to the imaginary axis of the ring.
What is an equatorial position?
A position on a chair conformation of a cyclohexane ring that extends from the ring roughly perpendicular to the imaginary axis of the ring. Equatorial bonds are oriented approximately perpendicular to the imaginary axis of the ring and also alternate first slightly up and then slightly down as you move from one carbon to the next. If the axial bond on a carbon points upward, the equatorial bond on that carbon points slightly downward. Conversely, if the axial bond on a particular carbon points downward, the equatorial bond on that carbon points slightly upward. Each equatorial bond is oriented parallel to two ring bonds on opposite sides of the ring.
What is a cycloalkane?
A saturated hydrocarbon that contains carbon atoms bonded to form a ring. Cycloalkanes of ring sizes ranging from 3 to more than 30 carbon atoms are found in nature, and in principle there is no limit to ring size. Five-membered (cyclopentane) and six-membered (cyclohexane) rings are especially abundant in nature.
What is an alkyl group? How are they named?
A substituent group derived from an alkane by removal of a hydrogen atom is called an alkyl group and is commonly represented by the symbol R-. Alkyl groups are named by dropping the -ane from the name of the parent alkane and adding the suffix -yl.
Stereocenter
A tetrahedral atom, most commonly carbon, at which exchange of two groups produces a stereoisomer
What is petroleum?
A thick, viscous liquid mixture of thousands of compounds, most of them hydrocarbons, formed from the decomposition of marine plants and animals.
What are the physical properties of unbranched alkanes?
Alkanes containing 1 to 4 carbons are gases at room temperature. Alkanes containing 5 to 17 carbons are colorless liquids. High-molecular-weight alkanes (those containing 18 or more carbons) are white, waxy solids. Several plant waxes are high-molecular-weight alkanes.
Do alkanes that are constitutional isomers have the same or different physical properties?
Alkanes that are constitutional isomers are different compounds and have different physical and chemical properties. As branching increases, the alkane molecule becomes more compact and its surface area decreases. As surface area decreases, London dispersion forces act over a smaller surface area. Hence, the attraction between molecules decreases and boiling point decreases. Thus, for any group of alkane constitutional isomers, the least-branched isomer generally has the highest boiling point and the most-branched isomer generally has the lowest boiling point.
How do we write structural formulas of alkanes?
Although the three- dimensional shapes of larger alkanes are more complex than those of methane and ethane (first two alkanes), the four bonds about each carbon atom are still arranged in a tetrahedral manner, and all bond angles are still approximately 109.5°.
What is a line-angle formula?
An abbreviated way to draw structural formulas in which each vertex and line terminus represents a carbon atom and each line represents a bond. In this type of representation, a line represents a carbon-carbon bond and a vertex represents a carbon atom. A line ending in space represents a -CH3 group. To count hydrogens from a line-angle formula, you simply add enough hydrogens in your mind to give each carbon its required four bonds.
Confirmation
Any three-dimensional arrangement of atoms in a molecule that results from rotation about a single bond. For any alkane (except, of course, for methane), there are an infinite number of conformations. The majority of molecules in any sample will be in the least crowded conformation; the fewest will be in the most crowded conformation.
Are alkanes soluble or insoluble in water (a polar compound)?
Because alkanes are nonpolar compounds, they are not soluble in water, which dissolves only ionic and polar compounds. Water is a polar substance and its molecules associate with one another through hydrogen bonding. Alkanes do not dissolve in water because they cannot form hydrogen bonds with water. Alkanes, however, are soluble in each other, an example of "like dissolves like". Alkanes are also soluble in other nonpolar organic compounds.
What is a saturated hydrocarbon?
Contain only carbon-carbon single bonds. Saturated in this context means that each carbon in the hydrocarbon has the maximum number of hydrogens bonded to it.
In a cyclohexane, where must the substituent groups be in order for the molecule to be the most stable?
Chemists have discovered that a six-membered ring is more stable when the maximum number of substituent groups are equatorial. In this position, the methyl group is as far away as possible from other atoms of the ring. When methyl is axial, it bangs into two hydrogen atoms on the top side of the ring. Thus, the more stable conformation of a substituted cyclohexane ring has the substituent group(s) as equatorial.
What are constitutional isomers?
Compounds that have the same molecular formula but different structural formulas. By "different structural formulas," we mean that they differ in the kinds of bonds (single, double, or triple) and/or in the connectivity of their atoms.
What are properties of Cycloalkane cis-trans isomers?
Cycloalkanes with substituents on two or more carbons of the ring show cis-trans isomerism. Cycloalkane cis-trans iso- mers have: 1) the same molecular formula and 2) the same connectivity of their atoms, but 3) a different arrangement of their atoms in space because of restricted rotation around the carbon-carbon single bonds of the ring.
What happens if you mix an alkane with a halogen and add heat?
For example, if we mix methane with chlorine or bromine in the dark at room temperature, nothing happens. If, however, we heat the mixture to 100°C or higher or expose it to light, a reaction begins at once. The products of the reaction between methane and chlorine are chloromethane and hydrogen chloride. What occurs is a substitution reaction... in this case, the substitution of chlorine for hydrogen in methane. If chloromethane is allowed to react with more chlorine, further chlorination produces a mixture of dichloromethane, trichloromethane, and tetrachloromethane. In the last equation, the reagent Cl2 is placed over the reaction arrow and the equivalent amount of HCl formed is not shown. Placing reagents over reaction arrows and omitting by-products is commonly done to save space.
How do we name alkanes with BRANCHED chains?
IUPAC names of alkanes with branched chains consist of a parent name that shows the longest chain of carbon atoms and substituent names that indicate the groups bonded to the parent chain.
How are alkanes named in the older system of common nomenclature?
In the older system of common nomenclature, the total number of carbon atoms in an alkane, regardless of their arrangement, determines the name. The first three alkanes are methane, ethane, and propane. All alkanes with the molecular formula C4H10 are called butanes, all those with the molecular formula C5H12 are called pentanes, and all those with the molecular formula C6H14 are called hexanes. For alkanes beyond propane, iso indicates that one end of an otherwise unbranched chain terminates in a (CH3)2CH- group. This system of common names has no way of handling other branching patterns; therefore, for more complex alkanes, we must use the more flexible IUPAC system.
What is the IUPAC?
International Union of Pure and Applied Chemistry
Cis-trans isomers
Isomers that have the same connectivity of their atoms but a different arrangement of their atoms in space due to the presence of either a ring or a carbon-carbon double bond. The prefix cis (Latin: on the same side) indicates that the substituents are on the same side of the ring; the prefix trans (Latin: across) indicates that they are on opposite sides of the ring.
What are stereoisomers?
Isomers that have the same connectivity of their atoms but a different orientation of their atoms in space.
What are the eight most common alkyl groups?
Methyl Ethyl Propyl Isopropyl Butyl Isobutyl Sec-butyl Tert-butyl
What is natural gas?
Natural gas consists of approximately 90 to 95% methane, 5 to 10% ethane, and a mixture of other relatively low-boiling alkanes—chiefly propane, butane, and 2-methylpropane.
Describe chlofluorocarbons
Of all the haloalkanes, the chlorofluorocarbons (CFCs) manufactured under the trade name Freons are the most widely known. CFCs are non- toxic, nonflammable, odorless, and noncorrosive. Originally, they seemed to be ideal replacements for hazardous compounds such as ammonia and sulfur dioxide formerly used as heat-transfer agents in refrigeration systems. Among the CFCs most widely used for this purpose were trichlorofluoro- methane (CCl3F, Freon-11) and dichlorodifluoromethane (CCl2F2, Freon-12).
Why are some haloalkanes important?
One of the major uses of haloalkanes is as intermediates in the synthesis of other organic compounds. Just as we can replace a hydrogen atom of an alkane, we can, in turn, replace the halogen atom with a number of other functional groups. In this way, we can construct more complex molecules. In contrast, alkanes that contain several halogens are often quite unreactive, a fact that has proved especially useful in the design of several classes of consumer products.
How do scientists typically write structural formulas for cycloalkanes?
Organic chemists rarely show all carbons and hydrogens when writing structural formulas for cycloalkanes. Rather, we use line-angle formulas to represent cycloalkane rings and represent each ring by a regular polygon having the same number of sides as there are carbon atoms in the ring. For example, we represent cyclobutane by a square, cyclopentane by a pentagon, and cyclohexane by a hexagon
Describe the reaction between oxygen and alkanes and cycloalkanes.
Oxidation of hydrocarbons, including alkanes and cycloalkanes, is the basis for their use as energy sources for heat [natural gas, liquefied petroleum gas (LPG), and fuel oil] and power (gasoline, diesel, and aviation fuel). The heat liberated when an alkane is oxidized to carbon dioxide and water is called its heat of combustion.
What does configuration refer to when talking about stereoisomers?
Refers to the arrangement of atoms about a stereocenter; that is, to the relative arrangement of parts of a molecule in space. We say, for example, that exchange of groups at either stereocenter in the cis configuration gives the isomer with the trans configuration.
Describe the haloaklanes classified as solvents
Several low-molecular-weight haloalkanes are excellent solvents in which to carry out organic reactions and to use as cleaners and degreasers. Carbon tetrachloride ("carbon tet") was the first of these compounds to find wide ap- plication, but its use for this purpose has since been discontinued because it is now known that carbon tet is both toxic and a carcinogen. Today, the most widely used haloalkane solvent is dichloromethane, CH2Cl2.
How do we draw alternative chair confirmations of cyclohexane?
Step 1: Draw two sets of parallel lines, one line in each set offset from the other in the set. Step 2: Complete the chair by drawing the head and foot pieces, one up and the other down. Step 3: Draw the equatorial bonds using ring bonds as a guide. Remember that each equatorial bond is parallel to two ring bonds and that equatorial bonds on opposite carbons of the ring are parallel to one another. Step 4: Draw the six axial bonds as vertical lines. Remember that all axial bonds are parallel to each other. Sets of parallel axial bonds are shown in color.
How can you draw a cis-trans isomer in 2D?
Substituents on the ring then either project toward you (that is, they project up above the page) and are shown by solid wedges or project away from you (project down below the page) and are shown by broken wedges. Occasionally hydrogen atoms are written before the carbon, H3C- to avoid crowding or to emphasize the C-C bond, as in H3C-CH3.
How do we name alkanes?
The IUPAC name for an alkane with an unbranched chain of carbon atoms consists of two parts: (1) a prefix that shows the number of carbon atoms in the chain and (2) the suffix -ane, which shows that the compound is a saturated hydrocarbon.
Describe the density of alkanes
The average density of liquid alkanes is about 0.7 g/mL; that of higher-molecular-weight alkanes is about 0.8 g/mL. All liquid and solid alkanes are less dense than water (1.000 g/mL), and because they are insoluble in water, they float on water.
What are the boiling points of alkanes like?
The boiling points of alkanes are lower than those of almost any other type of compound with the same molecular weight. In general, both boiling and melting points of alkanes increase with increasing molecular weight.
What is the most important chemical property of alkanes and cycloalkanes?
The most important chemical property of alkanes and cycloalkanes is their inertness. They are quite unreactive under normal ionic reaction conditions. Under certain conditions, however, alkanes react with oxygen, O2. By far, their most important reaction with oxygen is oxidation (combustion) to form carbon dioxide and water. They also react with bromine and chlorine to form halogenated hydrocarbons.
Are the bonds in alkanes and cycloalkanes considered polar or nonpolar?
The most important physical property of alkanes and cycloalkanes is their almost complete lack of polarity. The electronegativity difference between carbon and hydrogen is 2.5-2.1 = 0.4 on the Pauling scale. Given this small difference, we classify a C-H bond as nonpolar covalent. Therefore, alkanes are nonpolar compounds and the only interactions between their molecules are the very weak London dispersion forces.
What is the most stable confirmation of CYCLOHEXANE?
The most stable conformation of cyclohexane is the chair conformation, in which all bond angles are approximately 109.5°. In a chair conformation, the 12 C-H bonds are arranged in two different orientations. Six of them are axial bonds, and the other six are equatorial bonds. Axial bonds are oriented parallel to this axis. Three of the axial bonds point up; the other three point down. Notice also that axial bonds alternate, first up and then down, as you move from one carbon to the next.
What is the most stable confirmation of CYCLOPENTANE?
The most stable conformation of cyclopentane is the envelope conformation. In it, four carbon atoms are in a plane and the fifth carbon atom is bent out of the plane, like an envelope with its flap bent upward. All bond angles in cyclopentane are approximately 109.5°
What is an ALKANE?
The simplest type of organic compound. A saturated hydrocarbon whose carbon atoms are arranged in a chain.
What are the two major sources of alkanes?
The two major sources of alkanes are natural gas and petroleum. Petroleum and petroleum-derived products fuel automobiles, aircraft, and trains. They provide most of the greases and lubricants required for the machinery utilized by our highly industrialized society. Petroleum, along with natural gas, provides nearly 90% of the organic raw materials for the synthesis and manufacture of synthetic fibers, plastics, detergents, drugs, dyes, and a multitude of other products.
Do alkanes form a small number or a large number of constitutional isomers?
They form a large number of constitutional isomers. Though not on this list, CH4 can form 1 constitutional isomer.
How do we name cycloalkanes?
To name a cycloalkane, prefix the name of the corresponding open-chain alkane with cyclo- and name each substituent on the ring. If there is only one substituent on the ring, there is no need to give it a location number. If there are two or more substituents, number the ring beginning with the substituent of lower alphabetical order.
How do we derive IUPAC names of HALOALKANES?
We derive IUPAC names of haloalkanes by naming the halogen atom as a substituent (fluoro-, chloro-, bromo-, and iodo-) and alphabetizing it along with other substituents. Common names consist of the common name of the alkyl group followed by the name of the halogen (chloride, bromide, and so forth) as a separate word.
What is an aliphatic hydrocarbon? Why do we often refer to alkanes as aliphatic hydrocarbons?
We often refer to alkanes as aliphatic hydrocarbons because the physical properties of the higher members of this class resemble those of the long carbon-chain molecules we find in animal fats and plant oils. (Greek: aleiphar, fat or oil).
