Lecture 7 Environmental Organic Chemistry 1. Organic Chemistry Intro 2. dissolved and particulate organic carbon, Humic substances 3. DOC/POC distribution 4. Reactivity of simple organic molecules 5. Organic acids Pease read Manahan chapter 5 and chapter 29 (7 th Ed) for this week Organic Geochemistry of Natural Waters & Soils We will look at natural and contaminant organic molecules and summarize their behavior in the environment from the perspective of: - Functionality - aqueous solubility - volatility - environmental reactivity. Aqueous solubility is an important feature you should be able to recognize/predict since it affects reactivity and dispersion rate of organic contaminants. 1
Organic Geochemistry of Natural Waters & Soils Intro Let s talk organic chemicals. What is a common organic chemical you know of? Where does it come from? Examples of Common natural organic molecules in nature: simple organic molecules produced by organisms of synthesized naturally (i.e., methanol = CH 3 -OH) biomolecules (i.e., chlorophyl, amino acids, proteins) fossilized biomolecules (i.e., crude oil and coal components) 2
Common organic pollutants in nature are: pesticides/insecticides/herbicides/fungicides of various types (which can migrate from the application area) leaky subsurface fuel storage or transfer vessels. Biological waste products (untreated sewage, etc..). Gaseous contaminants from various sources, such as petroleum refining, chemicals manufacture, furniture manufacture, house painting. 1. Hydrospheric Organic Carbon Organic Carbon occurs in the hydrosphere in: natural and contaminant forms, as well as in dissolved and suspended forms: DOC = Dissolved organic carbon POC = Particulate (suspended) organic carbon TOC = Total organic carbon 3
DOC and POC concentration are variable in the hydrosphere but are generally higher in waters with high photosynthetic productivity: e.g., watershed water in forested areas the outflow of high photosynthesis lakes or swamps sewage outfalls 2. Structure, Nomenclature and Functional Groups: Organic molecules consist of a C-H backbone, sometimes plus other entities called functional groups, which can be substituted for either C or H. The molecule takes it's structural name from the number of carbon atoms it contains and how they are bonded. Each carbon atom can have 4 attached bonds C 4
More Structure, Nomenclature and Functional Groups: Organic molecules need not be associated with organisms. Many simple organic molecules are synthesized in nature by abiotic processes A few special types of carbon bearing compounds are not considered to be organic: these are the 1 carbon oxides (including and carbonates) and allotropic carbon (e.g., diamond and graphite). These are inorganic carbon. Nomenclature C, H - only molecules (aka hydrocarbons) Alkane all C-C single bonds ("saturated" molecule) Alkene Alkyne at least one C=C double bond ("unsaturated" molecule, has few H atoms in structure). If more than one C=C double bond, they are said to be "conjugated" if they are arranged like this: C=C-C=C-C=C at least one C C triple bond (also "unsaturated" molecule, has few H atoms in structure) Aryl special "unsaturated" molecule with "conjugated" C=C double bonds in a ring structure (like benzene = C 6 H 6 Benzene = = 5
Nomenclature: 1 carbon: Methane 2 carbon: Ethane (H 3 C-CH 3 ) Ethene (H 2 C=CH 2 ) note: when one of these molecules is 3 carbon: Ethyne aka acetylene (H-C C-H) attached to another molecule, it take the names "methyl", "ethyl" or Propane (H 3 C-CH 2 -CH 3 ) "ethylene" group for methane-like, Propene (H 2 C=CH-CH 3 ) ethane-like, and ethene like Propyne (HC C-CH 3 ) functionalities, etc.. Carbon atoms in these molecules are numbered, starting at the highest order functionality (see below), if one exists. Functional groups: These determine compound reactivity and water solubility. In general, more functionality = more soluble molecule = more reactive molecule. Functional groups typically set the name of the molecule: name formula Alternate names: a. Ethyl Alcohol (ethanol): CH 3 CH 2 OH (= ethane + alcohol) 1-hydroxy-ethane b. Propyl Amine: CH 3 CH 2 CH 2 NH 2 1-amino propane c. iso-propyl Amine: CH 3 CHNH 2 CH 3 2-amino propane the "iso" prefix denotes the location of substitution at the symmetrical center of the molecule. Common organic Functional groups: R= any carbon backbone R OH OR H 2 N OH R-OH R-O-R R-C=O R-C=O or "COOH" R-C=O R-NH 2 R-C-C=O alcohols ethers carbonyls carboxylic acids esters amines amino acids OH phenols pyrroles furans N Functional groups can be acidic or basic. Adding charge to an organic molecule generally makes it MORE water soluble and enhances Lewis acid/base interactions with metals in solution (i.e., chelation) The more basic (electron donor) functional groups an organic molecule has, the more interaction it is likely to have with inorganic ions. These can also interact with solid surfaces (e.g., Suspended load components of a river, sediments, etc...). One exception are halogen functional groups. "Halogenated" compounds do not act like lewis bases because these functional groups are very electronegative. O the points of the polygons are carbon atoms, with one hydrogen atom attached 6
2. dissolved and particulate organic carbon DOC = Dissolved organic carbon; includes many types of molecules, such as small simple ones and more complicated ones. Generally they are highly substituted with acid-base sensitive functional groups. POC = Particulate (suspended) organic carbon; also includes many types of molecules, such as weakly substituted hydrocarbons and refractory (less reactive) resides of large molecules, solid phase biological wastes..and the humic substances We will discuss hyperfine particulate (colloidal) POC later 7
Humic Substances interact with other elements and compounds in nature. For instance, they act as chelating agents for many metals in the hydrosphere and food sources for many aquatic microbes They affect the abundance in natural waters of micronutrients toxic metals radionuclides halogens humic substances contain long-lived free radicals which can reduce inorganic species such as Hg, Cr, and Pu. SEM image at 2000x magnification from www.hagroup.neu.edu/ 8
3. Solubility of Organic Molecules in water pure hydrocarbons have very low aqueous solubility substituted hydrocarbons increase in solubility by the number and type of functional groups it has. O, N and S bearing functional groups increase solubility particularly when the O, N or S can gain or lose a H + (and thereby give charge to the molecule). Halogen functional groups do much less to change solubility. 4. Reactivity of Organic Molecules Reactivity and decomposition products determine many things such as: the fate of biomolecules in nature what sort of hazard a pollutant might pose in the environment. Some relatively benign materials quickly decompose to hazardous ones and vice versa. 9
4. Reactivity of Organic Molecules Brief primer on organic molecule reactivity in nature. a). Reactivity of organic molecules in nature is somewhat different than in the organic chemistry lab. Two main reasons are: 1.the metabolic actions of microbes, which contribute to direct decomposition of organic matter and changes in system redox state 2.Photochemical oxidation/degradation 4. Reactivity of Organic Molecules b) Addition reactions: adding new functionality to a molecule. One important type is oxidation of an unsaturated hydrocarbon to alcohols or some carbonyl-based functionality. For example: H 2 C=CH-CH 3 (Propene) + oxidant CH 3 CH 2 CH 2 OH (n-propyl alcohol) 10
4. Reactivity of Organic Molecules b) Addition reactions: Molecules can be oxidized by interactions with O 2 or inorganic oxidizing agents (e.g., NO 3-, SO 4 2- ), or by microbes at aerobic conditions. Oxidation reactions are often catalyzed by bringing reactants together on the surface of a particle. Certain very selective oxidizing agents (and catalysts) do unpredictable things, but in general, the reactivity of molecules for typical oxidizers is alkanes < aryls < alkenes < alkynes, conjugated alkenes < non-conjugated alkenes. 4. Reactivity of Organic Molecules c) Substitution reactions: replacement of one functional group with something else. Nucleophilic substitution (nucleophile = lewis base): This major class involves interaction of a base with a positive or partial positive carbon atom in an organic molecule. 11
4. Reactivity of Organic Molecules c) Substitution reactions: There are a variety of mechanisms, most of which involve base interaction with an "acidic" part of the organic molecule while another base leaves the molecule. This usually happens at an area of partial or transition state positive charge on the organic molecule. Stable positive or partially positive charge on a C atom makes organic molecules more reactive to substitutions Environmental reactivity can be predicted from this. For instance: a. highly electronegative halogen atoms in an organic molecule might make that molecule more susceptible to attack than the same structural molecule lacking the halogens (e.g., with H atoms in their place). b. Other electronegative functional groups (e.g., -OH & -NH 2 ) become less electronegative at low ph (-OH 2+ & -NH 3+ ). c. In a polyfunctional molecule, the more and less electronegative elements work together to stabilize or destabilize that molecule to attack. 12
Conjugated systems distribute and stabilize their basic (π) electrons, making them more resistant to attack. Aromatic molecules (conjugated ring structures such as benzene and naphthalene) are typically even more stable. But depending on molecular structure, neighboring functional groups near a conjugated unsaturated system can contribute their basic electrons to the ring, either reducing or increasing the openness of the molecule to reaction. PCBs (polychlorinated biphenyls). These are an example of a relatively unreactive compound being made even more unreactive by substitution These pervasive contaminants are quite unreactive in the environment because the addition of Cl - groups to the benzene rings makes the π-electrons less reactive. The least reactive is decachloro biphenyl. It is important to note that many organic pollutants in the environment are persistent because they were designed to be that way. This affects strategies for cleaning up a contaminated area. 13
Reactivity of Organic Molecules d) Microbial decomposition: Microorganisms effect numerous chemical transformations of DOC and POC, some with quite complex reaction mechanisms. Some simple examples are below (see also next lecture).. Would these reactions be more or less likely to proceed with an electronegative functional group on the neighboring carbon? Distribution of Organic Carbon in natural water Abundance variations of DOC and POC are similar but POC is typically lower than DOC in most of the hydrosphere. Very clear (i.e., non-turbid) waters can have relatively high DOC and POC compared to inorganic solutes due to photosynthesis. Very polluted waters can also have high DOC and POC from the pollutants or from enhanced photosynthesis/ respiration (wastes from organisms) 14