CAS Common Chemistry API in Python

CAS Common Chemistry API in Python#

By Vincent F. Scalfani, Avery Fernandez, and Michael T. Moen

The CAS Common Chemistry API provides programmatic access to a curated subset, offering information on nearly 500,000 chemical substances.

Please see the following resources for more information on API usage:

NOTE: The CAS Common Chemistry API limits requests to a maximum of 50 results per page.

These recipe examples were tested on October 21, 2025.

Setup#

Import Libraries#

The following external libraries need to be installed into your enviornment to run the code examples in this tutorial:

We import the libraries used in this tutorial below:

import matplotlib.pyplot as plt
import requests
from pprint import pprint
from IPython.display import SVG
from time import sleep
import pandas as pd
from dotenv import load_dotenv
import os

Import API Key#

An API key is required to access the CAS Common Chemistry API. You can sign up for one at the Request API Access for CAS Common Chemistry page.

We keep our API key in a .env file and use the dotenv library to access it. If you would like to use this method, create a .env file and add the following line to it:

CAS_API_KEY=PUT_YOUR_API_KEY_HERE

load_dotenv()
try:
    HEADERS = {"X-API-KEY": os.environ["CAS_API_KEY"]}
except KeyError:
    print("API key not found. Please set 'CAS_API_KEY' in your .env file.")

1. Common Chemistry Record Detail Retrieval#

A CAS Registry Number (CAS RN) is a unique identifier for a specific substance. Information about substances in CAS Common Chemistry can be retrieved using the /detail endpoint and a CAS RN.

BASE_URL = "https://commonchemistry.cas.org/api/"
endpoint = "detail"
params = {
    "cas_rn": "10094-36-7"  # Ethyl cyclohexanepropionate
}

response = requests.get(f"{BASE_URL}{endpoint}", params=params, headers=HEADERS)

# Status code 200 indicates success
response.status_code

200

data = response.json()

# Print first level of the response data
pprint(data, depth=1)

{'canonicalSmile': 'O=C(OCC)CCC1CCCCC1',
 'experimentalProperties': [...],
 'hasMolfile': True,
 'images': [...],
 'inchi': 'InChI=1S/C11H20O2/c1-2-13-11(12)9-8-10-6-4-3-5-7-10/h10H,2-9H2,1H3',
 'inchiKey': 'InChIKey=NRVPMFHPHGBQLP-UHFFFAOYSA-N',
 'molecularFormula': 'C<sub>11</sub>H<sub>20</sub>O<sub>2</sub>',
 'molecularMass': '184.28',
 'name': 'Ethyl cyclohexanepropionate',
 'propertyCitations': [...],
 'replacedRns': [],
 'rn': '10094-36-7',
 'smile': 'C(CC(OCC)=O)C1CCCCC1',
 'synonyms': [...],
 'uri': 'substance/pt/10094367'}

Accessing Specific Data#

We can index into the data dictionary to access data from the response.

# Get Experimental Properties
data["experimentalProperties"][0]

{'name': 'Boiling Point',
 'property': '105-113 °C @ Press: 17 Torr',
 'sourceNumber': 1}

# Get Boiling Point property
data["experimentalProperties"][0]["property"]

'105-113 °C @ Press: 17 Torr'

# Get InChIKey
data["inchiKey"]

'InChIKey=NRVPMFHPHGBQLP-UHFFFAOYSA-N'

# Get Canonical SMILES
data["canonicalSmile"]

'O=C(OCC)CCC1CCCCC1'

Display the Molecule Drawing#

The images section of the data contains data formatted as Scalable Vector Graphics (SVG). SVG is an XML based vector format for defining two-dimensional graphics and has been an open standard since 1999. Using the SVG data provided by the API, we display the molecule drawing using the SVG() method from IPython.display.

svg_string = data["images"][0]

# Display the molecule
display(SVG(svg_string))
../../_images/dc483030f9cbc340140eaffa8fd3d845e828bd0d9291a6e8920e236e4a31115b.svg

2. Common Chemistry API Record Detail Retrieval in a Loop#

To obtain the data for multiple records, we request data for each CAS RN in a loop and save it to a list.

casrns = ["10094-36-7", "10031-92-2", "10199-61-8", "10036-21-2", "1019020-13-3"]

casrn_data_list = []
for casrn in casrns:
    params = {
        "cas_rn": casrn
    }

    try:
        response = requests.get(f"{BASE_URL}{endpoint}", params=params, headers=HEADERS)
        # Add a delay between API calls
        sleep(1)
        # Raise an error for bad responses
        response.raise_for_status()
        casrn_data_list.append(response.json())
    except requests.exceptions.RequestException as e:
        print(f"An error occurred: {e}")

# View the first item's summary
pprint(casrn_data_list[0], depth=1)

{'canonicalSmile': 'O=C(OCC)CCC1CCCCC1',
 'experimentalProperties': [...],
 'hasMolfile': True,
 'images': [...],
 'inchi': 'InChI=1S/C11H20O2/c1-2-13-11(12)9-8-10-6-4-3-5-7-10/h10H,2-9H2,1H3',
 'inchiKey': 'InChIKey=NRVPMFHPHGBQLP-UHFFFAOYSA-N',
 'molecularFormula': 'C<sub>11</sub>H<sub>20</sub>O<sub>2</sub>',
 'molecularMass': '184.28',
 'name': 'Ethyl cyclohexanepropionate',
 'propertyCitations': [...],
 'replacedRns': [],
 'rn': '10094-36-7',
 'smile': 'C(CC(OCC)=O)C1CCCCC1',
 'synonyms': [...],
 'uri': 'substance/pt/10094367'}

Select Some Specific Data#

# Get Canonical SMILES
cansmiles = [item["canonicalSmile"] for item in casrn_data_list]
cansmiles

['O=C(OCC)CCC1CCCCC1',
 'O=C(C#CCCCCCC)OCC',
 'O=C(OCC)CN1N=CC=C1',
 'O=C(OCC)C1=CC=CC(=C1)CCC(=O)OCC',
 'N=C(OCC)C1=CCCCC1']

# Get Synonyms
synonyms_list = []
for casrn_item in casrn_data_list:
    # Since the synonyms are a list, we need to extend the list
    # Instead of appending to avoid nested lists
    synonyms_list.extend(casrn_item["synonyms"])
synonyms_list

['Cyclohexanepropanoic acid, ethyl ester',
 'Cyclohexanepropionic acid, ethyl ester',
 'Ethyl cyclohexanepropionate',
 'Ethyl cyclohexylpropanoate',
 'Ethyl 3-cyclohexylpropionate',
 'Ethyl 3-cyclohexylpropanoate',
 '3-Cyclohexylpropionic acid ethyl ester',
 'NSC 71463',
 'Ethyl 3-cyclohexanepropionate',
 '2-Nonynoic acid, ethyl ester',
 'Ethyl 2-nonynoate',
 'NSC 190985',
 '1<em>H</em>-Pyrazole-1-acetic acid, ethyl ester',
 'Pyrazole-1-acetic acid, ethyl ester',
 'Ethyl 1<em>H</em>-pyrazole-1-acetate',
 'Ethyl 1-pyrazoleacetate',
 'Ethyl 2-(1<em>H</em>-pyrazol-1-yl)acetate',
 'Benzenepropanoic acid, 3-(ethoxycarbonyl)-, ethyl ester',
 'Hydrocinnamic acid, <em>m</em>-carboxy-, diethyl ester',
 'Ethyl 3-(ethoxycarbonyl)benzenepropanoate',
 '1-Cyclohexene-1-carboximidic acid, ethyl ester',
 'Ethyl 1-cyclohexene-1-carboximidate']

Display Molecule Drawings#

# Get SVG image text
svg_strings = []
for casrn_item in casrn_data_list:
    svg_strings.append(casrn_item["images"][0])

# Display the molecules
for svg_string in svg_strings:
    display(SVG(svg_string))
../../_images/dc483030f9cbc340140eaffa8fd3d845e828bd0d9291a6e8920e236e4a31115b.svg ../../_images/1e7bde96360cba6744881b59b890ccc10eeb40a000a61c2203c8cfb2ea1e4d2d.svg ../../_images/02320cce6221845bc605d80bb0f50b3ed1d23066d38225e299daec64a38f4e15.svg ../../_images/017052da4dbf1b3f8be198c3157c9350b3908efe69c7f632ca4c95199d23bc63.svg ../../_images/121b05ae45ac5e5eb45e6fcbc92ea7a2171bc3c7945c60b424cb4f53cf7b0ea7.svg

Create Pandas DataFrame#

df = pd.json_normalize(casrn_data_list)
# Select specific columns to display
df_subset = df[["uri", "rn", "name", "inchiKey", "canonicalSmile", "molecularMass"]]
df
uri rn name images inchi inchiKey smile canonicalSmile molecularFormula molecularMass experimentalProperties propertyCitations synonyms replacedRns hasMolfile
0 substance/pt/10094367 10094-36-7 Ethyl cyclohexanepropionate [<svg width="215" viewBox="0 0 215 101" style=... InChI=1S/C11H20O2/c1-2-13-11(12)9-8-10-6-4-3-5... InChIKey=NRVPMFHPHGBQLP-UHFFFAOYSA-N C(CC(OCC)=O)C1CCCCC1 O=C(OCC)CCC1CCCCC1 C<sub>11</sub>H<sub>20</sub>O<sub>2</sub> 184.28 [{'name': 'Boiling Point', 'property': '105-11... [{'docUri': 'document/pt/document/22252593', '... [Cyclohexanepropanoic acid, ethyl ester, Cyclo... [] True
1 substance/pt/10031922 10031-92-2 Ethyl 2-nonynoate [<svg width="292" viewBox="0 0 292 86" style="... InChI=1S/C11H18O2/c1-3-5-6-7-8-9-10-11(12)13-4... InChIKey=BFZNMUGAZYAMTG-UHFFFAOYSA-N C(C#CCCCCCC)(OCC)=O O=C(C#CCCCCCC)OCC C<sub>11</sub>H<sub>18</sub>O<sub>2</sub> 182.26 [] [] [2-Nonynoic acid, ethyl ester, Ethyl 2-nonynoa... [] True
2 substance/pt/10199618 10199-61-8 Ethyl 1<em>H</em>-pyrazole-1-acetate [<svg width="184" viewBox="0 0 184 62" style="... InChI=1S/C7H10N2O2/c1-2-11-7(10)6-9-5-3-4-8-9/... InChIKey=SEHJVNBWAGPXSM-UHFFFAOYSA-N C(C(OCC)=O)N1C=CC=N1 O=C(OCC)CN1N=CC=C1 C<sub>7</sub>H<sub>10</sub>N<sub>2</sub>O<sub>... 154.17 [{'name': 'Boiling Point', 'property': '130 °C... [{'docUri': 'document/pt/document/17362219', '... [1<em>H</em>-Pyrazole-1-acetic acid, ethyl est... [] True
3 substance/pt/10036212 10036-21-2 Ethyl 3-(ethoxycarbonyl)benzenepropanoate [<svg width="318" viewBox="0 0 318 101" style=... InChI=1S/C14H18O4/c1-3-17-13(15)9-8-11-6-5-7-1... InChIKey=HDGSRVUQMHJTDH-UHFFFAOYSA-N C(OCC)(=O)C1=CC(CCC(OCC)=O)=CC=C1 O=C(OCC)C1=CC=CC(=C1)CCC(=O)OCC C<sub>14</sub>H<sub>18</sub>O<sub>4</sub> 250.29 [] [] [Benzenepropanoic acid, 3-(ethoxycarbonyl)-, e... [] True
4 substance/pt/1019020133 1019020-13-3 Ethyl 1-cyclohexene-1-carboximidate [<svg width="163" viewBox="0 0 163 101" style=... InChI=1S/C9H15NO/c1-2-11-9(10)8-6-4-3-5-7-8/h6... InChIKey=OYGZRJWLVOOMBM-UHFFFAOYSA-N C(OCC)(=N)C=1CCCCC1 N=C(OCC)C1=CCCCC1 C<sub>9</sub>H<sub>15</sub>NO 153.22 [] [] [1-Cyclohexene-1-carboximidic acid, ethyl este... [] True
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