Introduction to Organic Compounds: Saturated Hydrocarbons, Exams of Chemistry

Download Introduction to Organic Compounds: Saturated Hydrocarbons and more Exams Chemistry in PDF only on Docsity! Chapter 11: Organic Chemistry. Saturated Hydrocarbons 11.1 Organic and Inorganic Compounds Historically, organic chemicals have been associated with living systems. This association originally derived from the belief that only living entities (i.e. plants and animals) could make organic chemicals. Indeed, until 1828 all attempts to prepare any organic chemical from exclusively inorganic reagents failed. Before we go further, we need to define what an “organic” chemical is, as compared to an “inorganic” chemical. Unfortunately, there is no ironclad dividing line between these classes of chemicals. Nonetheless, we have a few guidelines that work for the vast majority of compounds. (You’ll see few, if any, exceptions.) 1) Organic compounds always contain only p-block elements (Groups III-VII), at least one of which must be carbon. 2) Organic compounds almost always contain one or more C-H bonds. 3) Organic compounds are almost always molecules (as opposed to salts). Thus, all bonds are typically covalent in organic compounds. Methane (CH4) is the prototypical organic molecule. Stick drawings of methane and some other organic molecules follow. H C H H H C C C C C C H3C C CH3 O CH3 O2N H NO2 H NO2 methane (natural gas) 2,4,6-trinitrotoluene (TNT, explosive) acetone (nail polish remover) H3C C O H O acetic acid (vinegar) H3C CH2 O H ethanol (gasohol, beer) C C C C C C H H H H C C OH O CH3 O 2-acetylbenzoic acid (acetylsalicylic acid,aspirin) 2 Although uncommon, there are organic compounds that don’t contain a C-H bond. For example, CCl4 is almost always classified as organic. There are two reasons for this. First, the series CH4, CH3Cl, CH2Cl2, and CHCl3 are all organic and CCl4 is simply the final member of the series, and second, in nearly all respects it behaves chemically like the other compounds in this group. Likewise, although it is an ionic compound, [(CH3)4N][CH3CO2] (tetramethylammonium acetate), would typically be considered an organic compound. (We will get to how to name organic compounds later.) It is interesting that the very first “organic” compound prepared from exclusively inorganic reagents is now considered an inorganic compound. In 1828, Freidrich Wöhlers decomposed ammonium cyanate by boiling an aqueous solution of the chemical and obtained urea, a major constituent of urine. This experiment caused others to begin examining whether organic chemicals could generally be prepared from inorganic chemicals and it was quickly shown that there was nothing special about living systems in the synthesis of organic compounds. Some general properties of organic compounds include: 1) Like all molecular compounds, organic molecules typically have low melting points (in fact many are liquids at room temperature). This is because London forces and dipole-dipole interactions are usually the forces acting between molecules (p. 161). 2) They tend to have low molecular polarities. 3) Poor water solubility. Few organic molecules are readily soluble in water. (Although the 3 oxygen containing molecules on p. 1 are quite water soluble. We’ll see why later.) 4) Poor electrical conductivity. Few pure organic substances conduct electricity well. (There NH4NCO ∆ H2NCNH2 O H2O 5 so have 2 C-H bonds. Using this notation we can simplify some of the structures shown on the first page to Although you may find that it takes a little while to get used to drawing structures this way, you will find it makes your life much simpler in the long run. We now turn to some odds-and-ends of discussing structures. First the octane molecule shown earlier is a straight chain structure. Don’t take this as literal truth. In fact each carbon is tetrahedral, and if each CH3 in octane were grasped and pulled apart, a structure much like the drawing above (for octane, with the zigzag line) would result. When CxHy groups appear off the “straight chain” a branched chain molecule results. Thus 2-methylheptane would look like: Double and triple bonds are usually written explicitly. For example, propene can be written out in any of the following ways: O O2N NO2 NO2 2,4,6-trinitrotoluene acetone O H O acetic acid O H ethanol or CH3CHCH2CH2CH2CH2CH3 CH3 CH3CH(CH2)4CH3 CH3 or or (CH3)2CH(CH2)4CH3 C C C H H H H H H H3C CH CH2 CH3CH=CH2 6 There exists free rotation about single bonds. In other words, if you had CH3Cl and grasped the chlorine atom, the CH3 group would spin like a propeller. What this means is that in a flask containing octane, all of the molecules aren’t strung out like the picture, but rather they twist and coil and constantly reorient themselves. This will become important when we get to biochemistry. 11.3 Isomerism Molecules possessing the same molecular formula but exhibiting different structures are called isomers. One of the reasons we use structural formulae is to show this explicitly. We have seen two examples of this so far. The first was shown explicitly, ethanol (CH3CH2OH) vs. dimethyl ether (CH3OCH3) (p. 3 of the notes). Can you figure out the other (answer next paragraph)? There are actually several types of isomers but the only one we will be concerned with now are constitutional isomers. These are molecules that have the same numbers and types of atoms, but different atomic connectivities. We will see other types of isomers later. The other isomers are octane and 2-methylheptane (both are C8H18). The properties of isomers may be very similar to one another (as is the case for octane & 2- methylheptane) or they may be quite different (see Table 11.1, p. 335 of your book for ethanol vs. dimethyl ether). 7 11.4 Functional Groups The basic unit of organic chemistry is a molecule consisting of only carbon and hydrogen with only C-C and C-H single bonds. The variety found in organic chemicals largely derives from the replacement of hydrogen atoms with other groups, or the presence of C-C double and triple bonds. Collectively, these collections of atoms are called functional groups. A table (11.2) of important functional groups appears on p. 337 of your book. We will discuss most of these in Chapters 12 – 16 and all of them in the biochemistry chapters. There are a few points worth mentioning here. Molecules possessing the same functional groups frequently exhibit similar properties. For example, amines are molecules possessing an -NH2 group. Just like ammonia (NH3) is water soluble, CH3NH2 (methyl amine) and CH3CH2NH2 (ethyl amine) are water soluble. Both organic amines have unpleasant odors (actually worse than NH3) and form basic solutions when dissolved in water just like ammonia. Another feature of functional group chemistry is that the functional groups affect properties less as the molecules become larger. Let’s look at the solubility of alcohols in water as the organic groups get larger: Alcohol Solubility (per 100 g of H2O at 20 ºC) CH3OH any amount CH3CH2OH any amount CH3CH2CH2OH any amount CH3CH2CH2CH2OH 7.9 g CH3CH2CH2CH2CH2OH 2.7 g CH3CH2CH2CH2CH2CH2OH 0.6 g When a molecule contains more than one of the same functional group, the effect of the functional group on properties typically becomes more pronounced. Again let’s look at a series of alcohols. 10 All hydrocarbons of any type are non-polar. In general, they mix well with one another and poorly with polar substances. (Aromatic hydrocarbons have a somewhat greater ability to mix with polar substances than do similar aliphatic compounds.) 11.6 Naming the Alkanes and Cycloalkanes Your book has organized a set of rules nicely and all I’m going to do here is summarize them. These rules have been established by the International Union of Pure and Applied Chemistry (IUPAC). 1) All alkanes end in “–ane.” 2) Pick out the longest continuous chain of carbon atoms in the molecule. Anything attached to that chain, including other hydrocarbon groups, is a substituent group. The alkane chain is named according to the following sequence: 1 C = methane 2 C = ethane 3 C = propane 4 C = butane all other compounds use Greek prefixes to indicated chain length 5 C = pentane 8 C = octane 6 C = hexane 9 C = nonane 7 C = heptane 10 C = decane 3) If the alkane has substituents, begin numbering choosing the side of the alkane with a 11 substituent closest to the end. 4) Hydrocarbon substituents begin with the same letters as the chains, but end in “–yl.” (e.g. CH3– = methyl). 5) If there is more than one group attached, list them in alphabetical order. 6) When two identical numbering schemes exist, the substituent coming first alphabetically should be assigned the lower number. 7) With cycloalkanes, no number is needed if there is only one substituent. If there are two or more substituents bound to the ring, the one which comes first alphabetically is assigned the lower number. Examples: Name the following molecules: There are also some common (or trivial) names used for organic chemicals. One type is the old systematic method. You won’t encounter these names often in this course and from time to time you will do so outside of it. For example, rubbing alcohol is isopropyl alcohol using the old naming system and 2-propanol using the IUPAC method. The most important saturated hydrocarbon side chains with common names are: 2-methylbutane 3-methylheptane methylcyclopentane 1-ethyl-3-methylcyclohexane Cl Br 3-bromo-2-chloropentane 12 isopropyl isobutyl sec-butyl tert-butyl From what you’ve learned so far, you might have been able to guess that isopropyl alcohol is a molecule with an –OH group attached to propane. But where? The IUPAC name is 2-propanol so it is attached to the second (or middle) carbon. In general the prefix “iso” yields a group with the formula (CH3)2CH(CH2)n-, where n is a positive integer. Thus isopropyl has n = 0 and isobutyl has n = 1. A second type is the unique name. Much like the systematic name for water would be dihydrogen oxide, but no one ever uses it, such names exist in organic chemistry too. (Indeed, if you look at the alkane prefixes beginning at 5 the prefixes are based on numbers, but 1 – 4 are based on unique names that later became part of the old and new systems.) An example from a later chapter will do nicely here. Consider the molecule: C6H5CH3. From its structure you might guess its name was either methyl benzene or (as you’ll learn later) phenyl methane. In reality, it is never called either of these names. It is always called toluene (airplane glue). As we progress through this course, we will encounter a number of molecules that go by common, rather than systematic names. In nearly all cases you will see that the systematic name is large and cumbersome, while the common name is short and frequently easy to remember (Vitamin A vs. 3,7-dimethyl-9-(2,6,6- trimethyl-1-cyclohexen-1-yl)-2,4,6,8-nonatetraen-1-ol). CH3-C-CH3 H CH3-C-CH3 CH3 CH3-C-CH2CH3 H CH3-C-CH2 H CH3